Better games (people wouldn't be saying Halo was good if they had a proper port of BF2 or ETQW)
More types of games
Cheaper games
Better player base
Possibility to upgrade
Decent PC is better than any console
Edit: How did I forget mods? I wouldn't want to play Oblivion without them.
I was actually thinking about whether it would be cheaper if I had gotten a 360 instead of my computer and upgrading, then I realized how expensive games for consoles are. They pretty much stay at $60, so my friends only have 2-5 games. Over time, if they had as many games as I have for PC, their purchases would be WAY more expensive (I rarely buy PC games for the full $50 - usually get it for $40 or lower).