The modular aspects of 5E are mostly well done, or at least serviceable. The default settings leave a lot to be desired in the "plays like D&D" realm. You can make it play more like D&D, but you'll get there by monkeying with the modules and ignoring a great deal of the "advice" in the books. Cyclic initiative is from the devil.
Backgrounds and class "paths" are well designed, but lacking in execution for many of the core classes. The ranger is especially a missed opportunity, as it would have been trivially easy to have made the base class a non caster, and then added the casting back in on only some of the paths. On the plus side, some of the paladin options are both flavorful and fun. The bard is the best one they've ever done. (They finally realized that jack of all trades doesn't work in D&D. The 5E Bard is a full caster with a lot of skills.)
Backgrounds are rather bland and cookie cutter. They could have had a little more variety mechanically that would have let the flavor shine through more. Still, those are easy enough to spindle, fold, and mutilate.
Tome of Beasts (by Kobold Press) has monsters done better than the base Monster Manual. The two books together make a good mix, though.
Spells are organized poorly and on the cheap. Nothing wrong with alphabetical order for the main listing, but the spot in the books, the location of the lists, the lack of an index by wizard school, etc. leaves a lot to be desired.
There is a general sense of using traditional D&D terms in non D&D ways that contributes to the overall confusion. It's easy enough to figure out once you work at it, but unnecessarily troublesome for both experienced and new players.
Dexterity is too important. The game has shifted to make it more important, but the ability scores are mired in tradition. Either keep the tradition and the links that go with it, or change it and let the change run through the system. Halfway is just bad design.