it's hard to describe. for me, it's a summation of little things, but most notably the combat often feels sluggish. aiming feels really weird using a mouse when it was designed for control sticks. sometimes instead of a dedicated rebalancing effort, cheesy nerfs are implemented to compensate for the more efficient mouse/keyboard controls. for example, in the PC version of HALO, weapons were made less accurate.
graphics can feel lacking, or sometimes just different than what you normally see on a PC. in the (rather limited) games i've played, the console ports go for a lot of the effects rather than the sharper, harsher style that pc-only has been pushing toward. soft focus, film grain, bloom, lense flare (which was one of two things about Mass Effect that drove me NUTS, the other being the fking mako), etc.
and then you sometimes get bonehead stuff where it seems like they didn't care to even TRY to make a decent PC version with crap like mouse buttons not working (after 4 or so patches to boot)
uhg. i didn't mean to start this discussion again.