Well...
The combat, as I've said, is quite complex. You don't exactly need to calculate attack rolls and DC, but you have to know what does what. For instance, if you're playing "real" D&D, throwing a fireball will burn up your Web spell. This is the case in ToEE, unlike in NWN or other Infinity Engine games. Things like initiative count, especially since it's turn-based as opposed to the real-time combat encountered in IE games and NWN.
It's definitely more hardcore than NWN. If anything, NWN's implementation of 3E rules is pretty lenient, and not very faithful at that. If you ask most people, they'll tell you that NWN isn't "hardcore" at all. ToEE, on the other hand, is currently seen as the most "hardcore D&D" CRPG.
Plus, the game's based on a low-level module. You won't see all that many magical items (though my party still does end up pretty rich pretty early on), and the most advanced spells you are going to cast are of fifth level. Nope, no Timestop, no Shapechange, and no Dragon's Breath here. Forget about +5 Vorpal Holy Avenger, too. Mithral Field Plate +5? Zilch. (The best items I've seen so far are, well, masterwork items. But I haven't set foot in the temple yet.)
In spite of these, though, you might be surprised to see that ToEE has more skills and feats. Craft Wondrous Items, Craft Magic Arms and Armor, Brew Potion, Bless Water spell, Slippery Mind -- all of these, and more, are expected in any standard 3E D&D game. NWN doesn't have any of them. ToEE does. This ain't DD or Diablo or SPECIAL; it's more complicated, or, as some people might say, cumbersome. (I, of course, disagree. DD/Diablo systems are just too simple.)
I'll have to say, here, that I'm fairly familiar with D&D -- if not in pen&paper form -- thanks to having access to a lot of rulebooks and other material for research purposes, in addition to having played every IE game. So I delightedly jumped into ToEE and had little difficulty; I'm not a good example of how "hard" the learning curve is. Maybe someone else will have something to say? (Because most of the people I've seen reviewing ToEE are somewhat familiar with D&D already.)
Bottom line: just "a bit familiar with D&D" will get you by just fine.
