since it's a + thread, I will try to find my + sides of 4E, in my opinion that are worth keeping in some format.
1: at will spells(cantrips), this includes martial maneuvers. with scaling included.
2. removal of medium armor. It's hardly used in 5E and never in 3E.(outised mithral versions, but that is generally light armor).
3. everything is an attack roll.
4. Healing surges, much better than 5E HD healing.
5. Rituals.
6. AEDU system in theory, in practice it was awful, well the fact that every class had the same amount of everything is awful.
7. 5 Min short rest. 1hr is NOT a short rest. This cleared up later 3.5e feat(ure)s of once or X per combat usage.