In terms of what's in the books, probably most of it. In terms of what is actually used, very little.
What I mean by that is that the core of the game (combat in the first two tiers) got hundreds of thousands, if not millions, of hours of playtesting, and it really shows. And since that's the majority of what I need the game to support me with, most of the time it plays like a dream. And that's enough to make it the best version of the game I've played.
(Oh, and the rulebook in the Essentials Set is outstanding. I've found that the game actually plays better if you treat that as "the rules" and the core books as the first set of mega-expansions.)
But everything else is half-assed. Some of the niche character classes (notably Ranger and Monk) don't seem to work, the game as a whole starts to creak by 10th level and from there the support available plummets rapidly. The character sheet is just barely enough for people to use, but poor enough that a multitude of amateurs on here have done better. And the PHB index is remarkable in its awfulness - an auto-generated index would have been better, meaning that someone went in and carefully made it worse.
Support for the exploration and interaction pillars is spotty at best. Which is a description that applies to almost all of the DMG as a whole - it covers all the topics that a DMG 'should' cover (except one), but covers all of them in a very cursory way. There's basically nothing for an experienced DM here. And there's nothing for a new DM either - the one topic the DMG deliberately doesn't cover is how to learn to DM.
The game also seems to make loads of nods towards styles of play, only to fiercely undercut them - the Ranger and the Outlander have features that mean that players who want to interface with the "foraging for supplies" bit of exploration are instead handed an auto-win. Encumbrance is both too fiddly and too generous (and bags of holding too common) meaning that any idea of running a game of limited resources quickly disappears. And likewise the light cantrip and the ubiquity of darkvision means that any DM who wants light levels to be a realistic level of challenge gets to watch as their players trivially sidestep it.
The monster design is frequently uninspired and uninteresting - big bags of hit points that exist basically to allow the PCs to show off all their cool moves. I guess that's fine, since it generally means the players have fun, but it really could be better.
Then there are the adventures. "Lost Mine of Phandelver" is outstanding, and "Curse of Strahd" is very good (albeit deeply flawed). But the rest range from mediocre to abysmal (and not in a good way). Worse, very often the climax is also pitched at way too high a level for the actual PCs, which means they've had to build in a 'cheat' - Tiamat is weakened by passing the portal, the demon lords have beaten the tar out of each other first, the PCs have extremely powerful giant allies... Again, I guess the players have fun, but it's a great way to undercut their achievement.
I've said enough, so I'll stop before I get to the fairly dire rules supplements or Ravenloft...
It's incredibly frustrating - as I said, the core experience is probably the best of any edition so far. I've had great fun running it, and really wouldn't trade it for any of the other editions. But the flaws...