There's been quite a bit.
-CTF is back
-The Easter event now has an island to go along with it that only pops up during Easter
-Swamptown was redone, there's now an auction house alongside some other stuff
-There are rewards for logging in daily
-There's a currency you can earn by sparring called Spar Coins
-The Castle was updated (you might have already seen it, depending on how long you've been gone)
-Phase 1 of the new guild system was added.
-There's a temporary reward for towering until the new rewards are done - the top guild this season will receive an exclusive hat.
Now's a great time to return to the game, since the 10th anniversary event is still live. There are portals that take you back in time to old versions of the map, complete with quests and a ton of new content.
I'm sure I forgot some things that you missed, but these were the first things to spring to mind for me.