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Not blocking used games is the big one for me. Sure, it just means maintaining the status quo, but the status quo should be maintained in this instance. According to Shuhei Yoshida, the PS4 will play used games, at least. If Microsoft decides to block them and Sony doesn't, I think we will see a resurgence in market share from Sony.
I didn't really hear anything that was all that exciting at the press conference. Price is a major issue, as Sony learned with the PS3, but they didn't provide any information about that. I do like the ability to save snippits of gameplay so that I can admire my awesomeness later. Seriously; I remember one particular game of Madden I played over a decade ago, and it still puts a smile on my face. I was a varsity athlete in my younger days and played intramural sports in college, and the sense of "flow" and the joy of victory can be exactly the same as a physical sport when gaming is at its best. Plus I could be all, "Hey, Aaron, watch this," and make my friends watch me beat them again and again and again. Yeah.....that sounds fun.
The social aspect is cool too, I guess, but I don't really see anything that is all that amazing. They pushed letting people take over you game and get you through a hard part, which is something I would never, ever, ever do. Ever.
I got the feeling that this was more an exercise in stealing press away from Microsoft and Nintendo. We didn't even get a release window. I hope it really is awesome, and moves the state of the gaming art forward, but I can't really tell much so far.
Oh, to directly answer your question, 100% backward compatibility with PS3 games (and even PS2 if I'm going to dream) would be a very nice feature. Sony promised backward compatibility last time on the PS3, but only the first versions had it, and it was terrible even then. I hope they do better with this gen.