You know what excites me most about the next expansion. It's not pandas, it's not monks, and yet both are contributing factors. It's the hope of breaking that 10 character limit on any given realm.
I've tried making alts on other realms, meeting new people, trying to keep up with another community, and then losing touch after playing on my main realm exclusively for a few weeks. It just doesn't work for me, and as a result those other toons collect dust since the community is an important part of the game for me. Past the starting area anyway.
I always found the starting areas to be the most immersive and exciting parts of the game. I create a brand new character, spend a good hour languishing at the character creation screen to get that perfect look, and a name to match. All my hopes and dreams for that toon seem like they could happen tomorrow. Any rudimentary backstory and character personality is also fresh in my mind. All these things make those first 10 or so levels a lot of fun.
Then after neglecting my new toon for a while in favor of max level ones I lose that clarity. I still get excited about a lot of my character names; pretty much all of them thanks to some paid name changes, but any inkling of a personality gets muddled with my own and my character becomes me rather than me becoming my character. Paid race/faction/name changes also have a similar effect on a toon for me.
Let's steer back to the next expansion. The other thing I am really excited about has nothing to do with warcraft aside from being connected to the annual pass (nifty deal awarding a beta invite, mount and diablo 3 for promising to keep your subscription going for another year). I am excited by the prospect of the real money auction house in diablo 3. I have this neat idea that rare items will drop for me once in a while dabbling in diablo and I'll be able to sell them for a buck here and five bucks there that I will be able to spend on warcraft stuff like pets and mounts or perhaps just supplement my subscription.
It is an intriguing thought to think that the time I spend amusing myself with warcraft alts could be replaced by enjoying diablo alts and making money along the way to support my warcraft subscription. In reality I think I'd only get 1-2 months of playtime a year which I would probably spend on pets on mounts instead.
The rest of the new expansion stuff, nothing I wasn't expecting. New areas, new dungeons, new raids. I'll certainly take advantage of all that stuff, but as features go I've got the most interest in the additional character slots. That one tiny feature will have more impact on my long term attachment to the game than any other.