Battlegrounds are still permitted in WoW. Arena and World PVP still allows /follow and you can still IWT your way around in normal BGs, which should be just fine for DKs or Shamans.

As far as getting back into it, I'm not sure what kind of advice can be offered. There's not really much mechanics-wise that's changed for starting out a team. Except "phasing" has become more obnoxious with the addition of Cross Realm Zones.

You'll want to make sure you're set up for assist + Interact with Target for quests and looting {WoW has AOE looting now, which makes looting quest items easier}. Jamba has had a few nice additions since you last played, such as Advanced Loot, which will help.

Both DKs and Shamans are still fun to play and seem to do well in both PvP and PvE, though just about any combination can successfully do the new Scenarios.

They went nuts with daily quests in MoP, so before you get into any of that, I'd suggest looking at what you'll actually need before starting to work on rep. They also introduced Grand Commendations which you can get for MoP Factions at Revered, so you may want to just do dailies on your chosen factions on your main {or skip the obnoxious collection ones on your slaves} until you get the main to revered and then buy the commendation. Then all of your accounts will get the boost and you'll have fewer headaches with it. This will work for all of your toons as long as they're on the same BattleNet, so repping up will be faster on alts after your first team. You'll also want to note that Exalted rep doesn't really provide anything vital. Exalted will get you cosmetic stuff - pets, mounts. You only need to get to Revered for gear now.