Well, to my experience, the game works just fine - apart from Old Jed, of course - at least up until the second level of the Vale of Shadows's main dungeon, where I am right now. The only actual gameplay problem I have is that I level up a bit too slow right now: I'm about two levels behind of what I would be in a normal game. You could boost up the experience a little bit, especially on some tougher monsters: Everything gives about half the usual experience! And it's not funny when every undead, only excluding the lowest of the lowest of skeletons, has ridiculous damage resistances: When a Myrkul's sending won't go down until I throw literally everything I have at him, I'd love if he gave a lot more exp than he did. Not to mention mummies, which have always been pretty tough, and whose experience gain has dropped from several thousand to measly 75. Challenge rating my ass.
Edit: *After trying the zillionth time the entrance battle of the Vale's third level* How in the bloody hell could you say the game's "pretty easy for a part of 6"? What kind of a badass gang do you have?
Ehh, good thing there's Dalekeeper, which still works... *Gives everyone a 5,000 experience bonus*
Moredit: Kresselack's quest gives 700 xp? Seven bloody hundred from a quest that used to give thirteen thousand?! Yeah, this totally needs some rework here. It's a wonder how my bunch can gain levels at all without Dalekeeper.
Also Arundel doesn't seem to understand that I'm done with the Vale of Shadows, still telling me to go over there without furthering the plot.
Every other problem I have is purely cosmetic: The game still looks way too much like IWD2, although fixing this would probably be way too much trouble - though I suppose you could go and change the colors a bit to more IWD1'ish at least, or something. Also, it'd be nice if it kept IWD2 perfectly playable without messing it up: If BG1Tutu made BG2 unplayable, I'd rage. And, perhaps most importantly, the menu music desperately needs fixing: IWD1's rules, IWD2's sucks.
Also would be cool if I could still go to HoW and TotL after finishing the game, without taking a long hike to Kuldahar to talk to Hrotgar.
All in all, though, everything works just fine, apart from the absolutely sluggish exp gain, so I reckon you could start concentrate more on cosmetics from now on.