Since MoP released in late 2012 the development team has increased by at least 40 %, with another 40 % increase to come (This was stated prior to patch 5.4 at Gamescon 2013, so almost 3 years ago). If this is true, the dev teams goal size is 96 % larger than it was at the release of MoP. This means that they had to recruit, integrate and train newcomers during MoP, is that why WoD was so underwhelming in terms of patch content?
With Legion we are getting in my opinion a lot of new content, with primarly Artifacts, Demon Hunters, World Quests, Surammar, as well as the content we expect such as class reworks, tuning, raids and new pvp systems.
The question is, how much content will Blizzard be able to make in the comming patches? I remember Landfall (5.1) as a great midtier patch with story development in quests and scenarios. New Factions for the players to grind to exalted. And all this was made by a team that's half the size of today, how much content do you think Blizzard will be able to deliever this expansion?
Edit: Added sources.
http://www.gamespot.com/articles/wow.../1100-6422762/
http://www.mmo-champion.com/content/...-Arena-DLC-404