Just did a full playthrough post mountains patch with 2 friends.

The Meadows/Black Forest are pretty much the same with only the changes to how food works being obvious (for the better). Also trolls respawn now.

Swamp has the addition of a troll-like mob that allows you to make a new gearset. Decent addition that adds some challenge and potential grinding in the swamp area. Previously the only thing I really did outside of dungeons was farming surtlings for cores.

The Mountains update added a new dungeon/gearset for them. This set is a light armor alternative with a fairly cool fist weapon. Three or so new mobs as well. The mountains feel complete now.

The plains added a tar (slime) mob that is brutal to both the player and anything that isn't undead. Besides that its mostly the same.

Really enjoyed a new playthrough with friends. Theres new building items that add more options for endgame construction projects. Overall I'd say its worth coming back to if you want to enjoy the game again, but if you are expecting something new you should wait until mistlands.