I disagree on not running instances, I've been able to complete everything up to crypts with the 4 locks \ priest matrix. (had to respec COH to deal with the first boss in crypts, what an asshole)

Honestly, felguards are about as good as a budget warrior tank two levels under the mob when they are equal level to the mob.

TBH I think 4x lock\ Priest is one of the most awesome combos in the game. Levelling is a breeze, and the efficiency of AOE heals with lifetap is superb.

I currently do: 3 felguard locks with one affliction lock (I'm a big fan of malediction and need the extra aoe fear)

With COEX and dots, there are no trash pulls that can't be solved through creative pulling. Similarly, while boss fights take a real creative approach, they are for the most part soloable, even if it comes down to soulstoning everyone and pingponging them. (though fights that require you to tank two separate hard-hitting mobs can be problematic).

Can you run heroics with them? Not solo, no. But if you get a druid tank to sub in for one of the locks, your horizions are limitless.