Maybe it's just because I'm the kind of person who'd rather go backpacking than spelunking, but the underdark bores the pants off me. I find myself replaying the above ground parts of Act I over and over, trying different character builds, different goals, and that's fun. But the underdark is uninteresting.
Maybe this would be different without the level cap, if I was trying out new abilities and spells and whatnot, maybe that would make it more interesting. But right now, once you're there, you've already been level 4 for the majority of the playthrough, and every encounter, the solution feels pretty straightforward because you've probably solved the same problem the same way somewhere else three times already. It just feels repetitive, and I say to myself "why should I bother with this? I haven't played a halfling druid yet, let's go try that..."
