Reached 90 today on my ever present shaman from the vanilla days. These are some of my observations so far:
The scenery is great. Pandaria looks better than both Northrend and the Cataclysm zones and I enjoy just wandering around it.
Having just reached 90, the endgame content also looks to be much more promising than both of the previous expansions. My biggest issue with WotLK and Cata's philosophies was how they artificially funneled players into the latest raid tier with each patch, and in the process trivialize previous tier raids and dungeons so much that people simply ignore doing them. They seem to be handling things a lot better here, while keeping people out and active in the actual game world in the process.
In spite of being more of a return to form though, a lot of people are complaining over the game just getting old and being the same old thing, and they're very much right. They've added scenarios as an alternative to play now, but these are all very short, trivial and not very engaging. Seems more like they're just testing the waters with this to build more on it in the future as they've said scenarios are meant to replace group quests.
I think the most significant negative change to the game over the years is how Blizzard makes more and more design decisions with each xpack that end up diminishing player interactions in the game (you know, something very fundamental to a MMORPG experience). I don't really have anything against "easy mode" content in itself, but as it is now we've reached a point where Blizzard lets players "experience all the content" with no one in the group or raid ever having to utter a single word to eachother. The removal of group quests, and adding dungeon/raid finder systems in place of players manually forming groups with eachother chip away the situations that usually act as catalysts for socializing and nurturing relationships with the players in the world. This is a pretty fundamental part of what makes MMORPGs different, and without it you ultimately get a pretty hollow experience.