I must admit, I had no chance to actually playtest D&Dn by now, but reading through the material I am mostly very optimistic.
Actually there is only one thing I am really bothered about, and I am wondering whether nobody else is actually complaining about that topic.
I miss the FORT REF and WILL defense.
I understand that partly it is implemented in the new Saves (or old school saves depending on your pespective) against spells.
I can live with that even if I don't like it, at least for player attacks.
I think it is much more fun to have the player do their attack rolls.
I think monsters should only save against ongoing effects.
But one can easily revert these mechanics as houserules.
The main reason I miss FORT REF and WILL is, that there are no longer any options
for martial attacks left. Every attack will be targeted against AC.
I think it could be a great part of the game to direct different attacks against different defenses,
with different effectiveness dependig on the opponent
In 4 e it was nearly almost an interesting part of the combat to single out the worst defense
of a monster and make use of this knowledge.
I like this especially for the reason that different options to target different defenses of an
opponent brings significant variety to the combat with little more complexity.
(Mechanics like that could make even the fighter more interesting
and leaving the fighter as the "easy to play" class.)
Compare such instantaneous options to the effort of tracking a bonus lasting for one minute
(10 rounds) -(I won't allow anything like that in any of my games.)
Actually variant defenses don't have to be FORT REF or WILL, any thing could be used
But I would like at least 3 different defenses, which also could be affected by armor in different ways
therefore giving more options to different kinds of armor.
One even could connect different defenses to different weapon damage types in some way.
Maybe somebody is wondering why I am especially complaining about his topic.
The reason is, I feel myself quite confident to overhouserule nearly everything I don't like about a game (doin this for 25years).
But one thing I don't like to modify is the MonsterManual. I like to use the Monsters as they are
(and with them any published modules), because it's hard enough to prepare a game without such modifications.
Rules you have to modify only once to your liking.
But to adding homebrew stats to monsters can be quite laborious.