I was also in the Cata beta, and my experience was like so many others here: It was a cool experience, and I was the envy of my guild, but once Cata finally released to the public, it wasn't special to me and I lost interest in redoing all the stuff I had done (and re-done) for weeks prior in the beta. For that reason, I don't think I'll try to get in the MoP beta.

However, I will say that there was one advantage for me: I got to level a goblin and a worgen, which is something I probably would not have done, otherwise, because all 10 slots on what I consider to be my home server are full. I don't have a toon I want to delete, and although I have a few toons scattered on other servers, I've never enjoyed playing anywhere but my "home" server. It's just not fun without my friends and the support of my main(s). Because they are adding an 11th slot, I'll probably get to do a monk once MoP releases, so I don't really care about being in the beta. (I still think only 1 more slot is pretty lame, though. I would SO use more if they were available.)