I like the old school feel with a somewhat balanced rules system.
I dislike that balance between classes and damage output remains somewhat of a target. For example, having overpowered high level wizards is not a problem in my mind. And having fighters deal as much damage on average than wizards as appears to be the case in 5E, is uninteresting. I think that D&D is about the fighter winning against a wizard against all odds, because the fighter managed to sneak up on the unsuspecting wizard; it's about a party defending its spellcasters at high level because they're they ones that will win the day. I'm fine playing an underpowered fighter because that's what heroes are all about. I like that, when a party faces opponents with a caster, they see a threat immediately and want to focus it down; while the enemy will try to protect their caster as best they can. In 4E this concept was gone entirely. In 5E, it is somewhat back, but not as much as I'd like.