My issues with 3.X are a little more fundamental. I want to change up how defenses are handled. What the attributes are. How skills and non-combat situations in general are handled. How abilities and feats are distributed. What it means to be an Arcane caster vs a Divine Caster vs a Psionic character. I want to see clearly defined tiers with mini E6 style systems built in (ie you hit level 6, you stay there until you do something that actually pushes you over the edge from heroic into Paragon territory in game. You still progress, gaining feats/abilities, but as soon as you hit paragon, it's a noticeable jump in power, options, and how the game feels).

That's in addition to the standard "Bring Fighters up, force Casters down, and fix the scaling" issues that pretty much everyone generally agrees on in principle if not in implementation.

I also agree with Djinn on action economy. The full round action in general I find problematic. Personally, I've personally been toying with the idea of getting rid of full attacks, but instead gaining extra swift actions as you go up in level (likely coinciding with tier breaks). So you sort of get the same effect, but with a more flexible action type, making the effect of it much more noticeable.