hmm.
a. Have to agree with Concentration being too all encompassing. Could be my TSR era heart coming out, but sometimes it just seems arbitrary and even petty.
b. non mechanical, BUT: as I experiment with more(sub) classes, 5e's decision to list spells only by level , and class list is getting annoying as heck. esp in light of things like arcane knight's restriction to evocation and abjuration. and the Arcane tradition of Wizards. Abjuration, esp has very few spells, esp those appropriate to combat heavy adventuring. and it is a HUGE PITA to have to look up every spell's alphabetized listing to see what school it from!!!
b1, I'm also not a fan of the limited selection of spells available in the PHB. Yes, I know there are more in OTHER BOOKS, but that's my point- digging thru a pile of books to find something on the fly is irritating- for player and DM alike. This, more than anything else- save the race as class- is also why we ditched D&D for AD&D pretty early. Something I had forgotten about, until I started digging into B/X clones like OSE, lol.
c. I was actually a fan of 3e's methods of customizing and scaling up monsters. NPC classes, templates, monster advancement rules, the works. I DO love how 5e MM and Volos for example, statted out multiple...archetypes, I guess of many monsters. darn useful!
d. the proficiency bonus scaling is my other bone to pick. It really needs a wider variance in bonus, considering how much is tied to it.