Definitely one of the best expansion pre-launches Blizzard have done, definitely on a par with the zombie attacks for Wrath (as an aside, my biggest problem with Wrath was because the zombie event made it very difficult to get to the towns and cities for much lower level characters).
The class changes have been a little what the hell at times, with me trying to figure out what the hell has happened to my Paladin, though I do have to say that I prefer the much smaller attack pool because you can see what combinations will work better soon.
Demon Hunters are just plain vicious. Massive amounts of damage in a very short burst, especially with the eye-beam. Also if you can pull it off, it seems like if you do a rush attack from a glide, you extra damage. Maybe it just seems like it though, but it does seem like a viable tactic.
Never really used glyphs so its no skin off my nose.
Demonic invasions are kinda fun, but the rewards are kind of hit and miss, as I've gotten the same shoulder pads three times with the same character, while another has gotten gloves, boots and a belt.
Legion is definitely an expansion that got me back into WoW like Cataclysm did. So, I'll give the launch event, 4 and a half Pit Lords out of 5.