I'm surprised no one else is finding as much joy in this expansion as I have. That said, I don't know what the competition is with regards to other expansions in other games, so I can't quite say if this should be nominated as the best.
CLASS ADJUSTMENTS
The new skills opens up a lot of doors for the Warrior class (trying out a new Melee Mash Army, inspired by Shrek) thanks to Counter attack.
Paladin class is finally useful... Holy Armor is a great skill.
New Summon adjustments makes Summon style Mage FAR more fun and playable. I was able to beat the game in Impossible, 33% damage was done by Ancient Phoenix.
Dragon of Chaos is FANTASTIC now.
Black Hole allows damage mages to be more viable in the end game.
UNIT CHANGES
Rune Mages are buffed further in 1.3.1.
The Rune Mages open up possibilities for using level 5s more often, this helps out all classes.
Adrenaline system makes Orcs far more powerful and forces people to use new strategies when engaging them.
Engineers are a nice starting unit to help out the early game too.
ITEMS
Smoother opening for all classes due to better item distribution.
Inclusion of Gift Mod Pack items (see, this is probably why people lost some of that thunder, since a lot of people played the gift pack mod before the expansion pack).
EVIL NEW CHALLENGES
I don't know anyone who hasn't complain about Scrounger to some degree or admittedly to her power. The top of the Wizard Tower has boggled some, but not all.