So far from my experience playing the beta as well as my experience playing the pre-patch with all the class changes, invasions, tons of new content, there is absolutely nothing to be upset about!
I am going to go so far as to say, Legion just might (keep in mind im saying MIGHT), just might beat TBC as the best expansion ever made. I have seen / experienced nothing in the legion Beta to make me upset at WoW or Blizzard in any way.
How about you?
For you, what (if anything) could you possibly be upset about going into Legion?