What I dislike about 5E...
I dislike subclasses, and would prefer "paths", or something, where you have a list of abilities at, say, 3rd, 6th, 9th level or whatever. If you want to be an Eldritch Blade you had these abilities, but you could pick your 6th level from Battle Master and your 12th from Ghost Step Warrior.
I dislike the assumption that the attribute bonus will be equivalent to the proficiency bonus. One thing I appreciated about older editions was that the attribute bonus became a small contributor to the die roll at higher levels.
I dislike that monetary or physical wealth is irrelevant beyond, what, fifth level? No reason to wear more than rags, bribe people, donate to orphanages, build temples, whatever. Also, I can appreciate that people don't want the formulaic method to price / create magic items from 3e, but I think that was better in many ways.
I am greatly disappointed in the lack of wilderness adventure tools. No one on that team read any Burroughs, London, Verne, Defoe, or Stevenson. There is very little for Man vs. Nature situations, and what there is can be obviated by a fifth level party if not earlier. The B/X rules are better, and that was over 40 years ago.
I dislike that every utility spell requires concentration. The option of the wizard boosting the rest of the party, rather than just one character, is particularly difficult. Or, that a magician can't use all their spells to boost themselves and be a magical thief. I know people don't like that, but it's real handy if no one is playing a thief. And, if someone is doing that to just screw over another player then you tell the wizard to stop being a jerk.
I dislike every non-mundane thing being magical. I think there is plenty of design space for uncanny or preternatural abilities that are not necessarily vulnerable to a dispel magic or anti-magic shell.
I dislike the lack of guidance on social interactions. How much to bribe a guard? Can you bribe a guard? How do I get a guard that isn''t bribable?