I started doing arena on my sham as soon as they started up. I've done sham/rogue, sham/war, sham/DK, sham/DK/Ret, sham/war/ret, sham/war/rogue/holypally/DK, all as resto. Here's the issues I have come up with so far.
Ghost Wolf - I'm still not sure why this doesn't break all forms of CC. We have to spec into a instant shape shift, druids to not. Yet......
Rogues - Here's a great little story. With 500 res and 19k HP, I decided to duel a guildie rogue for a bit. He would kill me every time before stun lock was off. If I used trink then he could blind and start all over. Now sure my gear isn't full deadly, but the rogue had yet to run a heroic. He was in greens and blues, and only his helm was lvl 80. He had 2400 AP, no PvP gear, and no enchants. Now depending on crits, the geared rogue in heroic naxx gear would kill me in no more then 5 seconds. Now think about it like this. You are fighting a class that you cannot see, can stop you from moving, then stop you from casting, then when you do cast your heals take twice as long to heal for half as much. Since last week I have kept track. In 2s and 3s I have fought against teams with rogues 26 times and went 1-25. In teams without rogues I went 27-12.
DKs- This class is so OP it's not even funny, there's just no way that....... wait, what??? My grounding totem will eat their death grip??? Never mind, DKs are fine. (took me a bit to figure that out, but once I did their not so bad, just have to be good with your CD management)
Hex - Overall almost pointless. I feel that they should just combine hex and poly for a mage. No CD, doesn't heal, doesn't instantly break when you fart, and they can still move. For shams put the 30 sec timer and 45 sec CD on it, but make it instant. However, it isn't in the nature school, so you can use it to try and eat a CS but not get locked out of heals.
Purify - Great for PvE, ok for PvP. Usually it's only good for dispelling a locks curse. DKs and rogues stack 2-4 things on you and I rarely have the time to waste 1 GCD, much less 4. Probably wouldn't be so bad if it just removed EVERYTHING. Probably have to add mana or a CD though.
Overall healing in arena - Pretty pointless unless your in a 5s. Double DPS (mage/rogue/hunter/DK/Pally) is just too good to not do. Triple DPS isn't so bad either. Plus, if a team does want a healer, they should just get a pally because they will always guarantee 10 sec of uninterrupted heals, wear plate, and can stun. Shams have crap for mitigation, hardly any ways to get out of melee range, and can't really do anything against ranged DPS.
Overall healing in BGs - This is a lot more fun. Just make sure you stay in a group and you can really be helpful. Plus EB totem is always nice in a warsong.
What I'm really seeing different is the total lack of CC in arena. It used to be CC the healer and burn down the DPS, or CC the DPS and burn down the healer, and there was consequences for not doing so. It's not the opposite now, it's just a waste of time. You can kill a healer before you even worry about the damage you take from the free DPS.
Plus, as far as I can tell, there's no easy fix for this. Shamans need different ways to either lessen the incoming damage, or different ways to escape it. There just isn't any way to lower the damage that a certain class can put out. They have made a lot of the raid encounters dependent on a certain amount of DPS, if they lower classes like rogues (who are already low on the PvE side) and hunters, then those classes will not be getting raid spots.
I know shams aren't the only class having issues in PvP atm. And I know that there is shams that have done well in arena, and I'll tell you why. During S3 I did 2s on my mage as frost mage/ret pally. In a month of playing we capped out around 1770ish. We then transferred servers and in doing so battle groups. There we started a 2s team and hit 2200 in the first week, with only 9 losses along the way. Competition changes, you don't even think about pugging BGs on my shams server, because more then half the time you will be facing a premade. But on my mages server you can spam all day to start up a premade, but you probably won't get one going.
I don't want any of this to stop people from doing what they want. If you like to do BGs and arena then do. I'm not going to stop. The winning and losing doesn't mean as much to me as just having fun does. I like the smaller fights and I'll keep doing them, you should too.