Not exactly an exploit but a useful tidbit, if this belongs in guides feel free to move it.
Death Knights have the ability to "control undead". Black Rook Hold is FULL of undead baddies that you can control and have them fight for you, adding quite a chunk of DPS. I tested this in both heroic and mythic and it works for both.
Most useful I have found are the casters at the beginning, archers in the middle, lancers at the very end. If you happen to die your minion will either die along side you or will be released and will return to its spawn point wherever you picked it up.
This is extremely useful in beefing up your DPS. Obviously its just using a skill as intended and taking advantage of a situation but its quite nice.