The Xbox is pretty popular huh... With the exception of some old school consoles (snes, etc), I can't say I like consoles. Give me a decent pc anyday!
Higher resolutions, much better graphics.
The mouse... playing an fps with a controller is a joke. Or try and play a complex RTS
Customization: Counterstrike, Day of Defeat etc... they wouldn't exist if it weren't for the PCs ability to make mods for games.
Online play: So you have your xbox live now, your ps2 service... I think I prefer playing for free, bar my isp connection.
You can do more with a pc

word processing.. I don't really need to list these. Sure you can do a few more things if you put a modchip in your console, but it'll never be a pc.
To each their own, some will always like their consoles and what not. But the PC is a superior system in every way imho.
Oh yeah, and what I hate about consoles? Take Deus Ex. Brilliant game. Then they dumb it down so it can be played on a console - it sucked, and Ionstorm is pretty much bankrupt. Go figure.