Tank Like a Girl has a post on Heroics order, part I: the easy ones. Her assessment matches my experience. I know that some of the harder heroics are at least as hard as Karazhan. We’ve gone in with a group that’s geared in Karazhan loot and that has run together since level 1 and still had problems. The best advice I’ve seen for heroics is to treat them as a mini raid – bring a tank, a healer, two DPS with CC, and one DPS that can off heal. For more on heroic instances by difficulty, see this WoW Insider post and read the comments – for once they’re actually helpful.