The thing I want 5e to accomplish is "never more than two-rolls tops" to complete any action. Attack, then Damage. Grab, then Hold. Stealth vs. Perception. Diplomacy vs. Insight. Damage, then Saving Throw. I don't think any single player action should require more than this. I never want to return to the world of Grapple Checks, Trip Monkeys, or the summoner's pet horde of earth elementals.
A few other areas of 4e math works (or they got it to work via trial and error): how to handle large encounters. Monster math (post MM3). XP budgets (for those so inclined). I even have grudging respect for 1/2 level + Stat math (though, as stated, I'd love to see some way to use other mathematical advancements as well). I think some of these things, reworked into the frame of a AD&D/3e style class structure, could work wonders.