PC World's Matt Peckham has a great read on 15 reasons PC Gaming beats all.
I don't think that many of Matt's reasons are 'slam dunks', but here are two of my favorite excerpts:
Keyboard and mouse beats all.
"We've yet to see an interface as intuitive and broadly commanding, including (without question) Nintendo's Wiimote and nunchuk."Online PC matchmaking is free.
"...It's worth remembering that online PC matchmaking and multiplayer are, and always have been free. It's not a luxury item, it's not a special service, it's not a value proposition -- it's an entrenched and completely reasonable customer expectation."
Most of Matt's reasons ultimately boil down to the PC's superior flexibility over consoles. Many PC games ship with editors and built-in support for user-made content, for example, and hence offer a lot more potential long-term play. Finish off Neverwinter Nights 2 or your favorite first person shooter? Odds are there's a already a huge mod-making community out there with tons of free content waiting to be tapped -- for free.
On the down side, PCs are more complex, more expensive, and generally a bit more fickle than consoles. Not all games work as well from one system to the next, and some may not work at all.
However: