From reading the leaked docs, I'd say that I'd really like the Skill Challenge system, first off. I'd also like the 4 Defenses added and the "idea" of Fighter maneuvers/abilities. Fighters are just too weak at higher level and end up being retired if they don't just multiclass. Encounter powers would fix that, but I don't think they could call it the same thing. The XP Bucket that one poster mentioned is also great.