As far as the class selection goes, it really depends on your end-goal. If you're mostly interested in PvP then 5 Shammies is a safe bet. Any 4 of a DPS class and a healer is also a decent choice. Those types of setups can also do quite nicely in some regular instances, but they generally hit a brick wall when going into heroic instances.

If your end goal is to push the PvE limits, then you need a Tank/Healer combo to anchor your team and then go from there. I have no interest in PvP, so I started a Warrior/Priest/3 Mage team. That team did well, but later I decided to drop one of the mages for a warlock and it's been working out even better. That team can't aspire to much more than welfare arena points, but it's a blast to play in instances. Once properly geared, I'm hoping to do even the toughest heroics.