Don't really know why everyone is leaving bad reviews for a game that hasn't been released yet. Most complaints are around the level squish balancing and postponing of the actual release date.
a) Never in the history of pre-patches for WoW has the game been balanced, nor are those time periods aimed to be balanced. Pre-patches introduce the calibration for end-game content (i.e. level 60, not 50).
b) Blizzard has a pretty remarkable streak for never delaying an expansion until now. This is the 8th expansion, and I for one am glad they are taking the time to deliver a quality product.
I've essentially made this review in an attempt to offset the number of unnecessary negative reviews