The launch of MoM has been as close to flawless as I've seen in an MMO. I have encountered zero bugs and no downtime at all, save for when they brought the servers down to launch the expansion. The new areas are visually stunning, as you would expect (we are talking LoTRO). The quest design in new areas also seems subtly improved, they flow better than quests in some of the old zones. The new classes and the improved trait system are also very cool. Warden, especially, is a ton of fun.
However, so far my favorite addition is the new legendary item system. Legendary items have a socket / gem system similar to many RPGs. On top of that each epic item gets random abilities from a huge list when you have it identified, giving it a somewhat Diablo vibe. As you use an item it "levels up" giving you points you can spend to improve either core stats (such as DPS) or the other abilities the item possesses (your choice). It's like a trait system for magic items. Every ten levels, an item also gets a new random ability added to in when you have it reforged.
Leveling up my shiny new epic bow and watching the DPS slowly improve has been incredibly fun. I can see myself spending months finding the bow with the perfect ability set for my playstyle. I honestly haven't been this excited about hunting down new items since I played Phantasy Star Online. That the whole system is imbedded in a kickass full featured MMO doesn't hurt a bit.