This thread isn't feeling like much of a "celebration".
Anyway...
As someone who has DM'd about 200 sessions of 5E, it plays really easily from my side of the screen. Adjudicating actions is so, so simple. Everything is basically an ability check (with a few minor variations). If a check is situationally easier or more difficult, you just declare advantage or disadvantage. It makes rulings at the table quick and easy.
Most of the complexity is in the character and monster abilities. Characters seem to be built at about the right level of sophistication. I've had a handful of instances where players feel their character is too complicated or too simple, but that's been the exception.
On the monster side, I have some gripes about the design of complex monsters -- they tend to give DM's a lot of suboptimal choices. I wish complex monsters had fewer, more impactful options. Regardless, the 5E Monster Manual is one of my favorite D&D books, and I have good feelings about the other monster books, too.