Yeah, you can't just play games that are designed for PS3 due to being games that are built for the Cell CPU.
There will be no point in downloading any PS3 games on the PS4. This includes PSN games. They just would not work at all on your PS4.
So are they going to have 2 separate store fronts? The one they have now and the new one?
Are they essentially going to have all the DLC games rewritten to work on PS4?