Is it the keyboard and mouse? Or something else? For me it's "just because". I multitask while playing games. I like the adhoc community better than the canned environments of XBL, etc. And yeah, I like the complexity of my keyboard and mouse.
Graphics, Keyboard & Mouse, don't have to wait 30 minutes to play something
I use my ps3 so infrequently that EVERY TIME I want to play something, it requires a system update. Then by the time that's done I'm probably no longer interested in playing.
Being able to easily Mod my games or apply awesome fan-made ones without any hassle