Vehicle saving and customization. GTA4 had the half-measure of saving vehicles in the apartments' parking spaces. But if an NPC decided to drive off in it at the end of a mission, it was gone. So whether I can save and tweak to my heart's content as in SR3, or I have to save up and buy them like Sleeping Dogs, I want to be able to find a car I like, paint it, and keep it forever.
Dynamic combat. Open world games have shown us recently that they don't have to sacrifice when it comes to combat. Players have every reason to demand finesse. Niko Bellic was stiff as a board.
Traversal. Not just running and jumping, but climbing, squatting, rolling, vaulting, and diving. Anything less will be a disappointment.
Co-Op. The technological limitations are surmountable by now, as SR3 has shown.
Repeatable side missions. Making your own fun - which usually boiled down to getting a 5 star wanted level and seeing how long you could survive - is well and good. It's part of the joy of an open world game. But some other reasons to keep coming back certainly wouldn't go amiss.