My concerns are pretty much about game play rather than game balance. Game play is all about the ability to make decisions that matter and impact your success or failure whereas game balance is more about equality of outcome. My primary beef with Fifth Edition is a game is that it makes concessions to game play in order to have a more balanced game.
It feels like every opportunity they sought to bring the skill floor and skill ceiling closer and closer together. This touches on all points of the game from class design to spell design to monster design. Concentration instead of Sustaining a Spell, Hit Dice instead of Healing Surges, Neovancian Casting instead of Vancian Casting, Split Movement instead of positioning, Weak Combat Maneuvers, Champion being nearly as good as a well played Battle Master, relying overly on Advantage and Disadvantage, and monsters that are big bags of hit points are a few of my least favorite things.
I get it. All this stuff makes the game more accessible, but by removing a whole host of interactions they have cut down on a skilled player's ability to punch above their weight class. One of my favorite parts of playing and running Fourth Edition was that you could play a Fighter well in the same way you could a Wizard well in previous editions. I would like a higher skill ceiling.