The expansion has been announced. Diablo III: Reaper of Souls
http://www.diablofans.com/news/1975-d3s-expansion-reaper-of-souls-was-just-confirmed/ Some newness includes:
New class, The Crusader which is inspired by the Paladin
Level cap raised to 70
New Act, most likely takes place in Westmarch
New vendor that can transmogrify and enchant items
No limit on Paragon level
New Loot Run feature that lets you run a random dungeon with random weather, enemies and boss
I can't say I'm as excited as I would like to be, but I'm interested. The Crusader looks pretty "meh", I want my

ing Necromancer back. I'm disappointed we're only getting one new class as opposed to the two from Diablo 2 LoD. It looks like the new Act is similar to Act 1, which is my favorite from D3, so that's cool with me. I like the sound of loot runs, but I hope they don't water down the drop chance because it's just a short dungeon.
One interesting thing is that they are going to be buffing the legendary drop rate significantly. One quote from my link says this about doing an Act 3 run: "Now: 256 white, 399 blue, 275 yellow and one legendary items. Then: 73 white items, 266 blue items, 83 yellow items and 7 legendary items." Now they just need to change the stat rolls on certain weapons so I can stop getting intelligence on Manticores.
I'll most likely get the expansion, even with all D3's problems, I still had a good amount of fun with it. I definitely got my moneys worth.