For me it's the fact that it is pretty much a PS3 (which was easily my favourite console) just better in every way imaginable (apart from games library, but I'm sure it will get there).
UI - a beefed up XMB, better, faster, smoother, more apllications
Controller - I have absolutely no problem with the DS3, but DS4 is pretty much better in every aspect you could think of. I takes a couple hours to get used too the analog sticks, since they are smaller but after that it's an absolute dream. OK one thing. The battery is utter shit, 6-8 hours of playing.
Games - here I hope it will be a bit more of a quantum leap in some aspects like AI, physics, etc. especially considering how much potential the power of the PS4 holds, but for now the 1080p and either above 30fps or rock solid 30fps with tweaked physics is enough for me.
Overall I haven't played on my PS3 since Nov 29th with the expection of DBZ Battle of Z demo, which looked like a PS2 game and was overall terrible...
So it is happening...PS4 preorder.
Greatness Awaits!
