The one thing I can suggest from a 5 separate class team is: Don't bind your DPS / Tank / Healing spammer all to the same key. The sooner you get away from this the sooner you start achieving great things. I'm not saying it can't be done this way, but I can gaurantee you will wipe at least once when you get that Oh $#!+ moment and cannot react because you need to act separately. At least do Tank/Heals on one key and DPS on another.

This way you won't wipe to the first boss aggro dump because your tank was attacking and couldn't taunt cause you can't stop spamming heals which are on the same button as tanking and all your DPS was still throwing massive nukes while the tank had no say in the matter. (Ya, it's happened enough times I switched) If you start practicing individual control, you will probably find yourself refining a lot about your multi-class game.

I struggled through instances in northrend while leveling cause of this and now that I've switched I get invited to naxx10 as tank/3xdps/heals (not for all fights yet) with my guild.