My two gripes with 5e are 1) the death issue; I feel like it is too easy for PCs to stay alive. I would prefer that when you hit 0 hit points, you have X rounds and then you're dead. As it is, the odds are that you are going to stabilize without help.
2) The armor system. I preferred RM's concept that the heavier the armor, the easier you were to hit, but the less damage you did, while lightly armored subjects were harder to hit (able to dodge, duck, etc) but took more damage. Or just soak damage based on type.
While I feel that with the rest system they have done a brilliant job with dungeon crawl resource management, but I am less thrilled with it for a single encounter while traveling, for example. I have no idea how to fix it, so I'm not counting that as a major issue. I have just made most travelling encounters as night attacks so I catch them out of armor, and really ramped up the danger level because I know they're going to face the full spell and special ability arsenal of the party. Hit hard and fast because they can only deploy their best at a specific pace. It's not perfect, but it works.