Yeah I made a thread about this in Ruin battlegroup forums a little bit ago. It unfortunately got trolled into oblivion.
Anywho, yeah I would definitely say Horde needs more DPS classes that ENGAGE battles. Sometimes 1/2 of our WSG group is priests and mages. They're not bad classes, but they're definitely not good at starting battles. An ally hunter will open on them, an ally shaman will purge their PW:Shield or Mana Shield, and they'll die in a second. Then, when horde has none of these classes to do that task, namely Hunter/Rogue/Warrior, alliance will target me first thing, and then a lot of burst is lost and it goes downhill from there.
I would DEFINITELY say HUNTERS of all things are needed the most. Their range is ridiculous. They have a 41 yard range I believe, and I only have a 36 yard range. I've died so many times just to that alone. Not to mention the amazing damage output marksmanship has, especially combined with the 50% healing debuff. We have next to zero hunters anymore, and they're what we are in dire need of. Also, multiple hunters = dead targets.
2nd place on the list, I'd say would be Rogues and Warriors. Both, like hunters, have the mortal strike debuff. These classes are good at initiating combat, and good at allowing classes like mine to shine a whole lot more. Even though I can deal amazing DPS, I have no mortal strike. Without any of these three classes it gets harder for us to get past their healers. The main focus with Hunter/Rogue/Warrior is DPS, mortal strike debuffs, and the ability to initiate combat well.
3rd place, and I just kind of put this out there, would be Paladins and Warlocks. If you can get the gear for the appropriate specs, Paladin is a class you're pretty much never sad to see in a 49 battleground. Ret is amazing, they can flag carry very well with tank spec, and their healing is top of the charts imo. Warlocks deal good DPS, they are the 1v1 king of all kings @ 49, they can flag carry quite well with soul link if we happen to be short a tank, and of course fears are always appreciated.
Above all, play something that you enjoy, and play whatever class you want on either side. If you don't enjoy it, don't play it! But if you can help out, its much appreciated. Alliance usually always has a pretty good DPS makeup, and ours could definitely use some work. Hunters especially, we need hunters.