Depends on the class. I have 85s of paladin, warrior, death knight, hunter, druid, priest, and rogue. The lowest I can recall doing is about 6k, the most closer to 10k or so. I think the hunter and DK were the highest through leveling, while the others lagged behind to some degree. When I tanked on the DK, the numbers were higher due to vengeance, as that is a huge DPS boost over all the greens people are wearing.
That healer was just that guy who likes to stare at meters more than he should. They are the first to point fingers if something goes wrong and if nothing is wrong then the run is too slow due to poor DPS. I have all ten classes at 85 and never had a problem with reaching 8k+ DPS on any of my DPS characters. But then again I don't really care if people are struggling reaching 4k as my DPS is usually enough to make up for them. People should be hitting their minimum requirements for the dungeon, anything more is just gravy to make the run that bit smoother.