I have to ask. Did you play a Rogue for Cata PvP?
Agreed.
The only people who can honestly say MoP PvP was balanced are those who have only played the expansion on a private server running one of the later patches. Anyone who played retail knows very well the absurdity of Destro Lock, BM Hunter, Frost Mage, Arms Warrior, etc. and the ridiculousness of the gameplay changes that were made to PvP throughout the expansion.
Please tell me this is bait. Claiming that MoP is in any way more difficult PvP gameplay over other expansions, or that Shadowmourne equates to easy arena wins is ridiculous at best. Maybe if you're bad at the game. WoW has never been particularly difficult to play. In fact, that's the main thing that made it stand out in front of any of the older MMOs, that it was significantly easier to get into.
@Thread, I'm going to answer: Obviously WotLK is the best option for PvP on Warmane, because a larger community means that competitive PvP can happen - which means no Cata or MoP. TBC has significantly less spec and class diversity in comparison to WotLK as well. So, there you have it.