I cannot say anything on modding really (I tried it once, and I don't know the engine as much). It is the least covered of all engines as far as complete mods go, and the engine is quite unique. Only the
Planescape: Vengeance Total Conversion knows a great deal on the engine.
As for gameplay, let me add to that. I have despised the first 5 hours the game, since it is quite nasty, however as the story progresses the game gets better and better.
It was wrong of me to judge the game after that much gameplay, and that is why the game didn't sell as much as the Epic Hack n' Slash ToB did (the first few hours don't absorb you as much as the rest of the time).
I haven't played it quite through yet, since I hadn't had the chance to (I'd rather devote my time for modding than playing these past few years).
A mod that extra-planarizes ToB with a story at least half as decent as Torment (on an epic scale, but not munchkin scale) would be an excellent mod indeed.
-Galactygon