I like parties as well, but the only thing that's detrimental about party-based gaming is that you tend to balance the party in such a way that you end up trying out a lot of different builds in one game when you can customize your group as you see fit. It's the usual deal of having a warrior-type, a mage-type, a rogue-type, etc. In a single character game, you are more likely to be forced down one development path, leaving other builds for replays.
I like both systems, though, as long as they are implemented well.
