One problem I have with the game is that it is not Diablo II.
I mean, I thought the same as you did when I was first playing through it, but then I went back to play it again and it got boring really fucking quickly. I think that the reason for this is that in D2 you have four acts with completely different scenery (plus another act if you get the expansion).
I was thinking about this the other day, actually, because I'd just finished having a gameday with friends playing D2 all day and having a blast. In D2 you had easily-definable goals, quests and scenery. Someone can say "I'm doing an Andy run" and you know exactly what you're in for and what the scenery looks like. "I'm doing a Meph run" and the scenery is completely different.
The scenery in TQ isn't really that different, and there isn't enough variation in the weapons. The amount of weapon variation in D2 and the number of things that are viable builds without really knowing 100% about your character is something that I kind of took for granted. In TQ it seemed like I'd need to know a hell of a lot about the game before choosing my class/subclass and all the associated skills. I know I can refund using the Oracle, but you can't change your subclass. The classes in D2 were better defined, so you knew what you were getting in for.
All that said though I should mention that talking/thinking about TQ has got me wanting to play it again, so I guess that it is a good game that is suffering from the big brother syndrome. D2 was just so goddamned good that TQ has a lot to live up to, and for me it kind of falls flat in some areas compared to D2.