I've been playing for about twenty hours since the release on a vanilla server with 5 other people and this is what we have experienced.
The server is full vanilla set on HARD survival mode
1. Mob spawns are VERY low

• These mob spawns have been extremely low and slime spawns in swamps on a full moon are almost none(2-3 spawn) creepers/zombies/skeletons/spiders really don't spawn all that much and I've noticed I massive difference since 1.12 to now.
  2. New Villagers Trade's not resetting/not choosing closest work station
  -we have been really invested in the new villager mechanics, but it seems that only 1 librarian out of 8 reset trades even with a sufficient amount lecterns next to them and they all have access to a lectern but have not reset once.
  3. Chunk loading is bad.
  -It's not bad for me personally but for 3 people on the server, it makes it very hard to play the game.
  4. Bad omen.
  -Bad omen does not happen very often when doing raids.