Masterwork lets you customize who will invade you, and how many secrets/necromancers/megabeasts/titans/vampires/titans/demons/undead your world will have. Framerate improves a marginally, and it adds an auto-save for those times you get crashes due to extreme lag. There is also an assortment of lategame creatures you can add, for the hardcore defense artist. Or you can manually add a TON of enemy civilizations such as necromancers, orcs, night creatures and spider fiends that will siege you.
Succubus Dungeon seems pretty popular.
Perfect World lets you customize your world's geography (though it takes some trial and error to export).
Wanderer's Friend lets you craft needed items in Adventure Mode.
Mythical Monsters Mod lets you play as some awesome creatures.
Playable Naga Race... I'll let you guess what this one is for
Legends Viewer lets you graph population/historical statistics.
Modest Mod improves combat realism for bites and kicks.
Wizard's Tower adds more endgame to Adventure Mode.
That said, I think that the name generator for Masterwork is REALLY bad. Like, avengeing and forgeted are NOT words! And for some reason I can't get the vanilla tileset to function properly!
tl;dr - Masterwork lets you ramp-up the difficulty to how the game was in 2010, but with Tons more adventure mode & lategame content! If you're planning to play in a world for more than 200 hours or want lots of enemy civilizations then I strongly suggest it.