Though the distinction isn't hard-and-fast, components in this section are less concerned with improving the intelligence of creatures, more concerned with increasing their abilities and numbers. (Though hopefully, creatures remain intelligent and changes remain fairly low-key.) All components in this section are for BG2, BG2EE, BGT installs.
Make the starting dungeon slightly harder
This is a very low-key modification to the initial dungeon. It replaces the pathetic goblins with duergar (which I think has a better "feel" to it in any case) and it prevents you from sleeping more than once. (I allow you to sleep once because (i) it gives you a chance to choose your spells, and (ii) Imoen has some rest-related dialogue that I don't want to block.)
Improved Shade Lord
This component upgrades the Shade Lord, mostly by giving him additional magical powers (though his hit points are also increased). Unlike most SCS components, this one doesn't make any particular effort to stick with the vanilla game or with anything from PnP source material: like the Wolf of Ulcaster, the Shade Lord has his own unique abilities.
This component (which requires "smarter mages") makes demiliches more like their third-edition versions. The demilich Imprisonment ability has been nerfed: it now grants a saving throw vs. death at -5, although it also causes level drain even on a successful save. Demiliches are still 100% immune to magic, but they are not additionally immune to spells of L6 or above (this means that high-level antimagic works on them).
On the other hand, demiliches are now by-the-book 35th level wizards with significantly accelerated casting times (and the ability to use their Imprisonment ability in the same round that they cast a spell). (Only the demilich in WK has access to HLAs.) The net effect should be to make demiliches quite significantly more dangerous.
More Resilient Trolls
This component tries to turn trolls' regeneration from a minor book-keeping nuisance to a genuine threat. Trolls still collapse when reduced close to death, and still require fire or acid to finish them off - but a more significant amount of damage is needed (ranging from 2 hit points for ice trolls to 8 hit points for spirit trolls) and they will get up again (at partial strength only) much more quickly.
This component probably isn't for everyone. Some will find it tactically and role-playing-ly interesting; others will find it a chore.
Increase difficulty of level-dependent monster groupings
At various points in the unmodded game, BG2 chooses the strength of the monsters you face based on your level. Typically there are four encounters, and which one you get depends on your experience level.
This component causes the game to give you somewhat tougher choices; there are four options available. "Mildly increased difficulty" causes you to get a fight one step too high-level for you; "significantly increased difficulty" gives you a fight two steps too high, and "almost-maximum difficulty" gives you the hardest possible fight whatever your level (with one exception: see below). Other ability-dependent components are modified in a similar way to try to achieve the same effects.
The only exception to the general rule is the "spawn-undead" script, whose highest-level setting causes liches to spawn. SCS II liches are so much more powerful than vanilla ones that it is nearly suicidal to take them on at lower levels (at least without cheesy tactics), so even on the "almost-maximum difficulty" setting they don't spawn unless you have at least 1 million XP. If you really, really want a massively-hard-to-injure undead archmage dumping Pit Fiends and Horrid Wiltings on you at 8th level, install the fourth option, "maximum difficulty".
This component is rather like the "always tougher spawns" component of Tactics, but it's rather more customisable (and, since modding tools are much improved since Tactics was written, it installs in a way that's less likely to mess up other mods).
Improved Random Encounters
This component adds a little more variety and challenge to the "You have been waylaid by enemies" encounters. The main point of this component was not so much to make random encounters more difficult (though it does that too, to some extent) as to make them less repetitive - there are now about a dozen encounters, and the order in which they occur is somewhat randomised.
Improved D'Arnise Keep
This component, though not identical to the "Tougher Torgal" component of Tactics, is strongly influenced by it. It adds more trolls to the keep, upgrades Torgal a bit, and gives Spirit trolls some optional magical abilities. The main differences from the Tactics version are more detailed targeting for the spirit trolls, a wider variety of troll reinforcements (not just spirit trolls) and a (hopefully!) cleverer, but slightly less tough, Torgal (who, in this version, has not stolen one of the flail heads).
Improved Unseeing Eye
The original Unseeing Eye is actually a fairly ineffectual high-level spellcaster. This component shifts it into an Elder Orb (I assume that, just as the Eye grants magical sight to its blind followers, so it grants itself the magical use of its eyestalks even though strictly it's blind; I do assume its central eye is inoperative, though) and makes it virtually immune to damage (Demilich-level immunities) until the Rift device is used. Even once the device is used, the Eye will still put up a reasonable fight.
Improved Bodhi (Tactics remix)
This component is very closely based on the Improved Bodhi in Westley Weimer's Tactics mod. To quote from the readme to that mod:
Sigh. Bodhi is worth 91K XP -- more than almost anything else in the game. And yet she is a mere brute-force melee fighter who doesn't even drink healing potions. We make her more intelligent ... and give her some impressive pseudo-magical powers associated with vampires of myth and legend. Bodhi gains dominion over bats, the cold, the ground and the grave.
The SCS version of Improved Bodhi is updated to use modern WeiDU coding (to help with compatibility-friendliness) and SCS II AI scripting. She comes in two versions: a "hardcore" version which has the same stats and powers as Westley's version, and a toned-down version with stats closer to Bodhi's vanilla-game stats, less ferocious saving-throw penalties for her spells (-4 instead of -10) and no Deathly Shroud (Bodhi's lethal Fire Shield power).
You can fight Bodhi in chapters 3 and 6; in chapter 3 (again, following Tactics) she'll pull her punches, slightly more so in the toned-down version than the hardcore one.
Party's items are taken from them in Spellhold
This component does essentially what it says: when the party are captured in chapter 4, their equipment is taken from them and they have to survive the maze beneath Spellhold without it. (They have an opportunity to reclaim it before the final battle of chapter 4). A small amount of nonmagical equipment is available relatively quickly in the Asylum. Players (even solo players) might want to note that Imoen does carry the Enchanted Weapon spell.
This component is rather similar to a component of the "Big Picture" mod, but it's coded rather differently: most importantly, it creates a dynamic list of all the items in the game, which should prevent the "missing-item" bugs that caused some problems with the Big Picture component. It's compatible with the Big Picture version in the following sense: if you install the SCS II version after the BP version, SCS II dynamic-item-generation will be added to the BP version but that version will otherwise be left alone. Don't install the BP version after the SCS II version, though.
Improved fight with Irenicus in Spellhold
This component improves the clones summoned by Irenicus in the end-of-chapter-4 battle (see the Spoilers for details).
This component improves all sahuagin in the game (but especially the ones in the sunken city). No new creatures are added, and no special powers are granted, but the sahuagin statistics are systematised and generally improved slightly, and their scripting is smarter. (Various clues in the vanilla game suggest that sahuagin priestesses are cleric/mages; I disregard this and treat them as standard clerics.)
Overall, this component should be a bit more difficult than the vanilla sahuagin, but nothing like as powerful as Tactics' Improved Sahuagin.
Improved beholder hive
This component (adapted from Quest Pack) adds a few extra beholders to the hive in the Underdark, notably including a Hive Mother.
Prevent resting in the illithid cityThis component does just what it says it does: it prevents you from sleeping in the city. This is partly on role-playing grounds (it's blatantly implausible that you could rest there), partly on tactical grounds (a party at the kind of level that's likely to enter the city can protect everyone with Chaotic Commands, nullifying most illithid powers. This will force you to ration your protections a bit, and to be more creative.
Slightly Improved Drow
This component makes a number of small improvements to the drow of chapter 5 (it doesn't affect the drow of Sendai's enclave). It slightly increases the combat proficiency of drow soldiers, it adds a few extra drow to one of the Underdark ambushes and a few extra priests to the Ghaunadans; it improves the drow battlegroup defending the doors to the Underdark; it slightly optimises House Jae'llat nobles and adds one more noble and a few commoner priests and mages.
Notably, it also dramatically beefs up the defences of Ust Natha (as of version 8, this is an optional subcomponent). The place is a drow city, not the kind of place that even a 15th level adventuring party should be able to wipe out. Now, more defenders will continuously spawn. For the first twenty rounds or so, this will be disorganised, low-level groups. After that, the drow will get more organised and start sending serious war-parties against the party. Eventually truly powerful groups of mages and high priestesses will turn up.
Warning: this component is not intended to make the conquest of the drow city somewhat more interesting and challenging: it is meant to make it virtually impossible. It's probably possible to beat the drow and conquer Ust Natha without exploits, but it will be extremely difficult. The sane thing to do is to get out as soon as you can; even that will probably be a bit trickier than usual.
Players who do want a challenging but doable battle to conquer Ust Natha can make it more manageable at the console; see the Customisation section for details.
Improved Watcher's Keep
This component makes small changes to a few areas in Watcher's Keep to make them more tactically interesting. The changes aren't too dramatic; they shouldn't change the feel of the place much. See the spoilers section for details.
Improved Fire Giant Temple
This component improves various aspects of the Yaga-Shura part of chapter 8. Fire giants acquire some high-level abilities; elemental creatures are slightly improved and given better scripts; a couple of bonus monsters are added.
Improved Sendai's Enclave
This component improves a number of sections of the approach to Sendai. Drow spellcasters become slightly higher-level and can summon more dangerous allies; the drow war-parties are tougher and reinforced by other creatures; the illithid ambush is less of a joke. Also, the final battle with Sendai is modified: no actual creature in the battle is altered at all (though other components of SCS II will alter them) but the statues activate and attack in a more efficient fashion.
Improved Abazigal's Lair
This component (which has been drastically changed in version 6) improves some parts of Abazigal's lair by adding several new encounters and toughening up the existing ones. See the spoilers for details.
Improved minor encounters
This component slightly improves the difficulty of a small number of encounters scattered across the game (see spoilers for details).