A price drop may make me buy a PS4, free PSN Plus, and games I'm interested in. But, I'm undecided if I get a PS4 since I heard the Steambox is coming out soon, and the Steambox would also be a Linux PC, and a game console. Plus, there are plenty of games I can buy from Steam for lower prices.
I can also just buy a faster PC and a gamepad controller or joystick to have a game console experience on my PC by connecting a PC to a bigger sized monitor, or TV with HDMI, DVI or VGA video connectors.
I don't know when exactly I'll get one. (I half expect the PS4 to be sold out until well after christmas...) So probably mid 2014. (After inFAMOUS: Second Son releases.)
For me: Time. In time, there will be a big price drop, the games will not be made for the PS3 as often, and there may be new hardware upgrades available for the PS4. My PS3 still works great and I still love the games, so I'm not going to spend a big chunk of change for something I still don't really want yet. YET. In time I will though.