Aside from the usual bug-fixing and quest tweaks, the September patch added a new high-level region called Winter Vale and a revamp of the game's portal system. That's just for starters, too, since the Istaria devs are hard at work on the new Helian's Tomb mini-dungeon and quest series as well as the return of seasonal events.
Finally, there are several network and game client optimization projects in the works, including new launcher functionality and character management options. In short, there's never been a better time to return to (or discover) the world of Istaria. If you're curious, the game client is free to download, and you can also partake of an unlimited 14-day free trial or a feature-limited free-to-play option. Oh, and did we mention that you can build your own town and play as a dragon?
[Thanks to Paul for the tip!]