I cannot name an individual quirk so much note a holistic one: several aspects of the game have been simplified for the mass market it seems to me. For example, weapons and armor are do not offer enough mechanical variety and this diminshes cultural flavour options to me. Class and subclass features beg for more granularity too. A few more supernatural conditions could have been offered, and the monolithic proficiency bonus progression is just... lacking. Don't get me wrong, there's plenty good to be found in this edition, but I feel it falls short on closer examination.
I just had a thought: pehaps this simplifying approach was also a means to create design space for variants in campaign settings? FREX, homebrew wise, for my campaign world, I am considering changing the sorcerer and adding a witch class, and tweaking the whole weapons and armor tables. Looking at, say AiME there are wonderful tweaks to the rules to fit the flavour of the Middle Earth setting! So was that part of WotC's intent with the core rules?