I hate, loathe and utterly detest A/B testing.
I work for a company that has decided all features need to be A/B tested before being released to 100% of our user base. I work on the mobile product (specifically iPhone) and A/B testing has effectively guaranteed that the mobile product feels fractured, confusing and inconsistent.
On top of that I feel A/B testing discourages boldness and risk taking. You end up with all features becoming small, incremental, uninnovative changes developed over a short development period (why take the risk working on something large if the data proves negative?).
I miss the days of just making something awesome and polishing it up with internal betas.