For a site that's been stagnant for years, the new UI is a dramatic change. No wonder people are reacting negatively.

I've had the Pro account since pre-acquisition days, and I have to admit there was something strangely comforting in a predicable UX that never changes...

FB gets away with major UX changes from time to time (people seem to always hate them at first), it will be interesting to see if Yahoo gets away with this too.