The fact that PS4 will be so much easier to develop for and they already have so many great IPs and third-party devs eagerly awaiting it means there will be no shortage of games, so I'm on board from the start.
Combine that with rumors of Xbox using always-online DRM to block used games, and WiiU not being nearly as powerful, and PS4 could potentially run away with next gen like PS2 did.
For as much as I use my PS3 (every friggin' day for all manner of things), I'm not sure I need any more than 1 decent game at PS4's launch. After launch, a few great games would be fine. Or just give me The Last Guardian as a PS4 launch title.
It depends, if it has retro compatibility it's a day 1 purchase(or as soon as I can). But if it doesn't, and on top of that block used games, then I'll wait maybe a couple of years and buy a PS3 instead. --- "I find this rain quite pleasant. It feels as though raindrops are blessing our victory..."