Disc is ok but feeling much more vulnerable in MoP in pvp, but spec/gem etc for survival and play as defensively as possible and you'll mostly be ok. Get stuck in like the good ole days and you'll be dead quicker than you can reach your trinket hehe. The loss of manaburn is extremely noticeable now too
But disc is still much fun to play in pvp.
Disc is pretty powerful in arena but your lack of CC and escape abilities mean you'll be relying a lot on your teammates for peels if you're getting focused. I just recently started healing arenas on my monk and between roll, tiger's lust, diffuse magic, ring of peace, fort brew, life cocoon and revival you can survive or get out of just about any situation. It's a completely different story for priests though.