WoW's next expansion is Legion. I think it looks pretty interesting. A number of bloggers seem to think that it lacks a "game-changer", but I don't think a game-changer is really necessary.
The most interesting thing about demon hunters is that it is elf-only. This is pretty radical. For a while now, WoW has trended towards fewer restrictions. Classes have opened up to more races. For example, pretty much every race could be a monk or death knight. Demon Hunters represent a significant departure from that philosophy. Only high elves and blood elves can apply.
It's perfectly justifiable lore-wise. I think it's a better philosophy than the "all races can be all classes" ideal that WoW seemed to be heading to before.
The other interesting thing is that the Demon Hunter only has 2 specializations: one tank, one dps. Blizzard seems to be trying to move towards more distinctive specializations, and it will be interesting to see how the specializations for the other class weapons change.
Taking a page from LOTRO's Legendary weapons and FFXIV's Relic weapons, Legion will introduce an artifact weapon for each specialization. This weapon looks like it will level up and improve over the course of the expansion, including cosmetic options.
Retribution Paladins will get the Ashbringer. It will be interesting to see weapons the other classes get. One neat idea I've seen is that Holy Paladins can use 2H maces, and Uther the Lightbringer's hammer was a 2H mace. That would be an outstanding artifact weapon for Holy Paladins.
After years of shying away from class quests and class-specific content, Blizzard is also reversing that stance and re-emphasising classes with class orders. Basically, each class gets a shared location where they reform an organization from the lore. It looks to be somewhat like the garrison, with followers or champions that you can recruit as well.
I strongly approve of this direction. Class lore is one of the strengths of Warcraft, and embracing it again is a great idea. Having a special location you share with others of your class is also good, and it will be interesting to see what locations are chosen.
Whatever is chosen, I expect the druid area to be a 24/7 dance party.
Story / New Areas
The story seems pretty solid. The Legion is one of the great enemies in Warcraft, and is always fun. There seems to be less emphasis on orcs (other than Gul'dan) which is good. The new areas all look interesting.
Legion seems to be bringing some extensive class changes. Blizzard has hinted that they want to try and differentiate the specializations more, and make them more focused around the class "fantasy". For example, Survival hunters are apparently becoming melee with a pet, which is pretty crazy. Beastmaster will be ranged with a pet, and Marksmanship will be ranged without a pet.
It will be interesting to see what they do with the other specializations.
Overall, I think the expansion looks pretty good. While there are no "game-changers", there looks to be lots of meat to delve into. If Blizzard can get the expansion out in a timely fashion, say November or December, I think they have a winner on their hands.