This is sort of a spiritual successor to 20 observations from a leveling tank, if you'd like a more tank-flavored look at leveling groups. This outing is a more generalized approach, possibly because I take a more observational role in my groups whenever I'm healing, like Jane Goodall among the ungemmed and unenchanted chimps.
1. DPSers are enormously indifferent to aggro in early dungeons. You're not healing one tank -- you're healing four. Five, if nobody bothers to stomp the mob making a beeline for you.
2. Early dungeons aren't necessarily good training for everyone involved. I wouldn't go so far as to say they're a terrible experience, per se -- they're quick, easy, and a good way to build confidence for new players -- but the usual mechanism by which players are encouraged to behave themselves (ugly death) is a remote possibility at best.
4. If you zone into a group as a replacement healer, it's not uncommon to find players in a really foul mood. If you're replacing a healer who got kicked for failing to keep the group alive, a healer who got kicked for what the group thought was failing to keep it alive, or a healer who left due to ongoing idiocy, chances are pretty good that everyone's had to run back from the graveyard at least once already.
5. That's how you find out that healers aren't usually able to impact a group's mood the way a tank can. I always say hello to people when zoning in and chatter a bit, and I am a lot more likely to get replies as a tank than a healer. Angry or silent groups tend to stay angry or silent when I'm healing. (I've always hoped it isn't because I suck, but you never know.) The only explanation I can offer is that healers are unique in playing a largely reactive and observational role within a group, and maybe there's not a lot you can do that will affect people's experience within the dungeon apart from simply keeping them alive.
7. Tanks are largely indistinguishable from each other at the 5-man level. Despite the endless fights about raid balance, good tanks in 5-mans are just good tanks, period. Class mechanics have very little to do with who provides a smooth run and who doesn't, and they're all roughly the same to heal.
8. Everyone should play a shaman at one point or another, if only because your totems provide an excellent education on which buffs overwrite each other and which ones don't.
10. The ability to crowd control without aggroing the group has resulted in some interesting ways to experience dungeons. And by "interesting," I mean half of Vortex Pinnacle can be skipped with the right mix of classes.
11. There are a lot of competent players out there who quietly make their groups a lot better. To all the hunters out there who trap mobs trying to eat me, death knights who Death Grip errant caster mobs over to the group while they're nuking me afar, mages who sheep, retadins who toss on a shield and grab everything when the tank goes offline, and everyone else who does stuff like this on the down low expecting (and usually getting) no credit -- thank you. I usually only have time for a quick thanks in party chat before the tank who can't be arsed to do any of this himself has sprinted for the next pull.
13. People are still doing that "drop every dungeon you don't like" thing. This seems to happen most frequently with Stonecore and Vortex Pinnacle. These are the two highest-level dungeons you can get before attaining an average ilevel of 304, and so players tend to get them over and over and over again while they're trying to meet the magic number.
14. The toughest dungeon to heal while leveling is Slave Pens. The Coilfang Ray's Psychic Horror ability is usually good for a few wipes, either because you're the one feared while an undergeared tank is taking massive damage or because someone else gets feared into another group. The Coilfang Soothsayer's Domination is pretty much the same deal, especially if the mind-controlled player is the tank or a DPS who proceeds to beat the crap out of you. As for the bosses, Rokmar's bleed can be nasty to heal on a tank without decent avoidance, and even on normal, Quagmirran does a nasty amount of damage, not least with Acid Spray.
15. Guild groups come in only one of two flavors. Either they're great people and it's easy to see why they're playing together, or they're there to roll on everything they can and make complete and utter douchecanoes of themselves.
17. If you're gearing a healer and trying to get valor points from 5-mans, queue on Tuesday or Wednesday. People tend to run their mains earlier in the week and their alts later. A guildie clued me into this shortly after patch 4.1's change to the daily heroic, and it was a godsend to my shaman with 60K mana in blues.
18. Healing is the best way to become the in-game equivalent of a psychic. After healing for a long time, you develop a sort of sixth sense for what a group's fate is likely to be. After the first pull, you know how well-geared the tank is, the rate at which he/she is taking damage, the rate at which the DPS is killing the mobs, and if anyone is going (or needs) to crowd control. All you need is that one pull, and you'll have a pretty good handle on exactly how that group will experience a dungeon.