I'll be surprised if this doesn't wreck the in game economy. Since you can buy gold in the RMAH items in the regular AH will probably be massively inflated. I just don't see anything good coming from this. Thankfully you can just ignore it all and play with your friends.
It ignores whether the game is designed and balanced around RMAH.
It could have disastrous effects on grouping with random people. If drop rates are based around the idea you can easily get items off the AH then that'll fuck up single and co-op with friends.