The important thing to remember is that this long drought is because of how much of a failure WoD was. You can tell they released 6.0, then 6.1, and realized nobody liked the content, and they didn't even know what they were going to do with it. There are plotlines that are just cut off at the knees or completely ignored all over WoD, and it's because of how badly the expansion went over with the community. From what Blizzard has said, 6.1 was ready to go at launch but they staggered it to buy some time to finish up 6.2, and start full development on Legion. WoD is clearly an experiment that failed for Blizzard, and it's no secret.
I almost think WoD was just developed and launched to give people some insight into the leaders of all the Orc Clans before the Warcraft movie came out, and they were featured on the big screen, but that's verging on tinfoil hat theory. The problem Blizzard had was they tried to speed up expansion releases, which meant rather than being fully committed to developing content patches, they got 2/3 of it ready for launch, and then launched 1-2 patches during the expansion before basically moving on. People feel like they are getting cheated with expansions now, especially after WoD. MoP had a lot of great content--that was poorly paced, especially SoO--but also a lot of crap people didn't want to deal with.
On top of that, they have done a really miserable job assessing why certain things are successful, so we're stuck with a lot of crap that people hate because it missed the target. A prime example of this is the fact that Blizzard claims the WoD questing system and partially Garrisons were developed because Timeless Isle was a bit of a hit. The problem is that it wasn't a hit because it was an endless grind, it was a hit because it had a few cool rewards, somewhat dynamic content, and most of all was a great way to catch-up undergeared characters. They realized that and introduced that part into Taanan Jungle, but by then people were already turned off to the entire expansion--plus, having a huge section of the map visible but just walled off is not a good practice. They took notice of the fact that people wanted more exploration, and it shows while questing. The problem is they then had backlash for not having Flying, because once you were max level it was hugely inconvenient to get anywhere (they tried solving this with that stupid Mage Tower building in Garrisons, but everyone realized just about every other building added real benefits). Then when they finally relent, and add flying, it has a massively time consuming requirement to get it, so people are still pissed.
I hope with Legion Blizzard decides to take a bit of a break from quickly releasing content to move to the next expansion, and makes this a 4, 5, or even 6 content patch expansion. It's back to Burning Legion and such so they have plenty to work with, and there's always room to throw in an obligatory Troll or Old Gods patch that adds some new and interesting content. With the setting being islands around the Maelstrom they can seriously just keep making crap up--since the lore indicates most sane people in Azeroth ignore that region like the plague. I really want to get back into WoW, but WoD is just not fun. It's time consuming, poorly paced, and just about everything relating to Garrisons is miserable.