If you do only 5-mans, you don't really get to taste the sweet nectar of being put on SA duty. 5-mans need to be relatively simple, with obvious mechanics or mild punishments to assure a PuG can expect to do well without prior training in the intricacies of the fight. Oculus is the only dungeon to do anything close to this with the drakes, but that's a vehicle fight; everyone's doing something different (and horrible, in Oc's case).
My favorite Special Assignment to this day is staying in the back for Four Horsemen. For those who didn't do Naxx, there are four horsemen in four corners, and if any one of them has no target within 40-ish yards, they start wiping the raid, so two dudes must stay near the two weak-ass spellcasters while the other two are tanked like normal. I got to stand in the back because I am crazy awesome rad, and have skills to pay ALL the bills. Also, I can heal myself.
When the raid leader picks you for a Special Assignment, he is saying "You. You are more than your numbers, impossibly huge though yours may be. You are clever and adaptable, so you can be TRUSTED with this". DPS are never trusted; they are the shady mercenaries in the cheap bar to any raid leader, whose loyalty is bought with the promise of loot, which leads to higher numbers. Being trusted is one of the two greatest honor a DPS can receive (the other is Vigilance).
I hunger for these encounters, where instead of "this fight is a tank-and-spank except for this, that, those, and these" it comes out "us 6/7/8 guys just do our job like normal, but YOU chosen ones need a special briefing. Report to spec-ops". Saurfang is case in point; men chosen not for their crazyrad deeps, but their kiting skill. It brings in mechanics we as DPS never otherwise touch, changing our experience up drastically. So here's to Blood Beasts, to Brain Rooms, to Arial Command Units. I hope ICC has more of the unexpected in store for us.