Fight to the finish!
Now Throne of Bhaal fails to meet this bar for three reasons:
1. Lack of exploration. This was one of the major pluses of the first two games. Even though the main quest required attention, you pretty much could not continue on without hitting at least a few side quests, and if you wanted to grab them all, you'd be playing for a very long time. The final chapter has none of this outside of the Watcher's Keep. I'll admit that the Watcher's Keep was awesome, but even with that in place there wasn't enough to keep the fresh feeling.
2. Broken game mechanics. There is something seriously wrong when all of the strategy used in the former games is replaced with spray and pray tactics. A few battles being nothing but desperate dogpiles I can dig, but half of the battles? Sorry, but that's just wrong. The battles that aren't like that feature enemies with an AC that is impossibly low, so it feels like you are fighting a fanfiction dream. Why did the final boss need to use game breaking mechanics? I don't care if she's bathing in the essence of a god, it wasn't any fun.
3. Crappy endgame portion. Now this part is just my opinion. The narrative was just fine and dandy as it has been since the first game until the end. It seemed like the "twist" was just a cop out because they couldn't come up with an epic way for you to fight your father. There was absolutely no buildup for that twist in either of the former games (because the character it revolved around didn't exist until the expansion) so it felt like an epic juicy steak funneled into nothing but a limp noodle of an ending. (Note: None of that includes the two endings you get based on alignment and choice, those were fine.)
Even with these things, that game is by no means terrible. In fact, even if it was I'd still play it just because of how incredible the soundtrack is. There are plenty of good things about this that make it worthwhile. Every single bhaalspawn fight is incredibly epic, and unlike some of the other ridiculous fights in the game, actually require strategy and pacing. These few fights are fantastic and stand for what the entire game was about. Unfortunately it was some of those fights in between that ruined it.
So there is a mix of fun and frustration, but honestly it will all be excusable if some wonderful modder makes a better ending.