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1 - Most older-than-ps3 PSN purchases will still work 2 - ps2 may work (certainly could technically but they may want to encourage PSN purchases of re-released ps2 games, plus HD releases) 3 - more hd releases, including of ps3 titles (down the line... such as 1080p versions of 720 games) 4 - some ps3 games, especially less intensive ones, quickly ported for ps4, like Castle Crashers, PS MInis, etc. 5 - Gaikai is used for streaming new and old games, may require subscription or be part of PS+
just guesses - but I hope they include as much B./C as possible -- doing it in software (thru the store) just brings them more money, makes their library larger and requires the user to have less devices.