Okay first off, being new to the site, but not the game my opinion may or may not be received well.
I took a break from the game due to a deployment to Afghanistan during the WotLK era and stayed away from the game a bit until just after Cataclysm was released. I made one toon level 80 while I still played but he was the only one. I am glad to have done it though (so the race to hit 85 wasn't as bad) and I was capable enough to get him rolling again relatively soon after I returned.
I think the opinions to be had about cataclysm have their merits (Both good and bad ones) but unless you can get a good running start in MoP, Maybe the idea of returning just before the release wouldn't be a bad idea, especially if you want to experience the content right away or try to get into raiding as soon as possible.
That's just my opinion though... Also with the scroll of resurrection being revamped, you could get a boost on another character to level 80 and just have that 'one more' toon that maybe you always wanted to try with the new system in place or the new one ahead.
Take it easy!