These components remove, move or tone down the powers of a small number of magic items. It's not at all systematic (there are mods out there which go through almost all the items in the game and reduce their powers) but it reduces the extent to which a party can coast by on a few über-items.
Make elemental arrows like BG2 (BGEE,TUTU)
The arrows of Fire, Ice and Acid in BG are seriously powerful in terms of damage done - much more so than their equivalents in BG2, presumably because the designers chose to tone it down a bit. This component applies a similar tonedown to BG.
Replace +1 arrows and other projectiles with Fine ones
This component replaces the weakest magical projectiles with non-magical ones of high quality. The main point is to make the "Protection from Normal Missiles" spell less ineffective when used by enemy spell-casters.
Replace many magic weapons with fine ones
This replaces most of the standard (i.e. non-named) +1 weapons in the game with non-magical equivalents: they still give you +1 to hit and damage but they don't give you +1 to initiative and they can't harm creatures that are immune to normal weapons.
This is mostly a cosmetic change: I wrote it because otherwise there are implausibly many magic weapons lying around the Sword Coast. It has two significant effects on the game, though. Firstly, +1 weapons cost a lot less (about a third as much). This mostly works against you (you generally find +1 weapons on enemies and sell them). More seriously, the iron plague affects "fine" weapons. I actually quite like this (feels more realistic, and makes the iron plague more a part of the game) but if it's just going to annoy you, don't use it: you can choose to have Fine weapons without the effects of the iron crisis. (It's clear from what you're told in-game that the crisis affects all iron (even iron not forged from Nashkel ore), but some people feel this is outweighed by the annoyance of having your high-quality weapons break so much. It's up to you.)
My alpha version of this component (for BG2) changed +2 weapons too, but that turns out to have unwanted effects on game balance - +2 nonmagical weapons are great to use against mages with the Protection from Magic Weapons spell running, and the protagonist is probably immune to nonmagical weapons in ToB and needs to be affectable by at least some opponents.
This component applies to all versions of the game (though on BG2 installs there is no iron crisis, of course). For technical reasons there are actually two separate versions (one for BG2, one for BGEE/TUTU/BGT): the right one for your install should be selected automatically.
Reintroduce potions of extra-healing (BGEE,TUTU,BGT)
This places smallish numbers of potions of extra-healing at some of the temples (see here for details) on the Sword Coast. It's mostly here because some of the enhanced encounters give potions of extra-healing to the bad guys, and it's rather unfair if the player can't buy them too.
Remove Arrows of Dispelling from stores
Arrows of Dispelling are arguably a little overpowered (they are all-but-eliminated in BG2, though some mods reintroduce them). This component gives you the choice of removing all, or all but five per store, of the Arrows of Dispelling from the stores throughout the game.
Remove the Shield of Balduran from the game (BG2,BG2EE,BGT)
I looked fairly extensively for ways of scripting beholders to deal with the Shield of Balduran without resorting to cheese, or for ways of toning down its powers while keeping it recognisable, but I had limited success. Ultimately, the character equipped with the Shield of Balduran is nearly invulnerable to beholders, pretty much irrespective of what strategy they use. This component (which, as always, is optional) removes the shield entirely from the game, leaving beholders as a challenge again.
(More recent versions of SCS have given beholders the power (after a few rounds) to steal the shield using their telekinesis power. It's possible that this means this component is not essential any more; I leave it in place anyway, since it's already written.)
Remove the invisibility power of the Staff of the Magi (BG2,BG2EE,BGT)
The Staff of the Magi allows the wielder to go invisible, at will and without losing any time to do so, and this is famously overpowered. This component just removes the invisibility power from the Staff.
Move Vhailor's Helm into Throne of Bhaal (BG2,BG2EE,BGT)
Vhailor's Helm is not absurdly powerful relative to some of the items that can be found later in the game, but it's a bit much for something that can be bought for cash at the start of chapter 2. This component moves it into the early part of the Throne of Bhaal expansion (see the spoilers for details).
Move the Cloak of Mirroring into Throne of Bhaal (BG2,BG2EE,BGT)
You can't get the Cloak until later in the game; still, it's hugely powerful and could be argued to be unbalancing at the time it's introduced. This component moves it into Throne of Bhaal (see the spoilers for details).
Move the Robe of Vecna into Throne of Bhaal (BG2,BG2EE,BGT)
Similarly, the Robe of Vecna - one of the most powerful items in the game for a spellcaster - is overpowered for something that can be bought in chapter 2, and so this component moves it into Throne of Bhaal (see the spoilers for details).
Make the Healing and Resurrection powers of the Rod of Resurrection into separate powers (BG2,BG2EE,BGT)
Having a magic item which not only restores its target to life but also heals them fully is extremely powerful. This component mildly tones down this power, by giving the rod separate resurrection and healing powers. It can still restore its target to life, but they have only one hit point; it can also cast a Heal spell on living targets.
Change Carsomyr so that its dispel-on-contact power gains a saving throw (BG2,BG2EE,BGT)
Carsomyr (the holy paladin sword) dispels all magical protections instantly on hitting its target, which drastically changes the flavour of many battles (notably those with mages and priests, and to a lesser extent with dragons). This component (which was borrowed directly from Demivrgvs' and Mike1072's Item Revisions) grants a saving throw against this effect, which makes it useful but less transformative.