The following is a list of somewhat random observations I have collected after several expansions' worth of tanking for low-level groups.
1. Don't take shortcuts on trash packs. The time you save sneaking past one of them will be eliminated by the time you'll lose when someone blunders into them and dies.
2. Someone will almost always blunder into them and die.
3. Despite common complaints on the forums, the vast majority of players are actually really nice people who are perfectly willing to tolerate mistakes and the learning curve. The actual occurrence of true, unforgivable jackasses seems to be about one per five groups, although this depends on when you're queuing.
4. Two seconds are required for warrior tanks to apply Rend to one mob and then Thunder Clap it to the rest of the pack with Blood and Thunder. This is apparently one second too long.
5. If you pull like a ferret on meth and your group's constantly scrambling to keep up with you, you'll always have that precious second before they catch up.
6. Doing this if you're undergeared is a great way for your healers to hate you. By the way, they're right. How do you reconcile these competing directives? You can't!
7. The worst pull in 5-mans while leveling is the first trash pack in Auchenai Crypts. It can be pulled cleanly if the group is willing to cooperate and withholds DPS until you've gotten the caster(s) out of the next group's range. However, this has not happened to me since my main tanked the place during The Burning Crusade.
8. Actually, I would nominate Auchenai Crypts as the worst leveling dungeon for a tank. Uninspiring loot, mobs that spawn other mobs out of your reach requiring immediate attention before they take off for the rest of the group, only two bosses, and large trash pulls all add up to a fairly irritating experience.
9. Most problematic DPS while leveling: frost mages. The same stuff that keeps them alive while they're out questing -- slowing and freezing mobs -- is the same stuff that can make positioning mob packs safely a hassle. Runners-up: Moonkin and elemental shaman who get overexcited about Typhoon and Thunderstorm.
10. On that note, is it just me, or are pugged hunters a lot better on average than they used to be?
11. Players have grown used to the relative lack of mob incapacitates, disarms, and stuns in 5-mans from Wrath through Cataclysm. The presence of these mechanics in classic and BC dungeons tends to throw them for a loop.
12. Spending most of a pull stunned or disarmed while a DPS pulls aggro is not a good feeling. Being lectured by said DPS player for not doing your job is worse.
13. Reading a group's mood is tough, and the line between a group that thinks you're pulling too quickly and one that thinks you're pulling too slowly is more fragile than you'd think.
14. There is nothing worse than a player who has a tank of her own at level 85 and can't shut up about it.
15. The dungeon in which you are most likely to lose aggro is Hellfire Ramparts. This is before a host of new DPS death knights in gear that is usually better than yours realizes what Blood Presence does.
16. You can avoid a lot of potential trouble (although I don't exactly know why or how this works) by apologizing for a fairly inconsequential mistake early in the dungeon (for example, "I'm sorry about that, got stunned and couldn't taunt that mob off you"). Maybe it makes people feel more comfortable if someone's expressed respect for their time or the willingness to own up to a mistake, but it's very rare for me to have any problems with a group when it's happened.
17. Blackrock Depths is probably the single best dungeon that Blizzard has ever created, though I would also nominate Scholomance, Magisters' Terrace, and perhaps Halls of Reflection. However, it's a pretty complex experience with quests all over the place -- and players who want all of them -- plus an ill-defined set of bosses that will complete the dungeon's different stages. Don't queue between levels 50 and 58 unless you've got at least two hours on your hands, and kill all the things. Eventually, whatever completes the section of the dungeon you got will die.
18. Playing a druid tank has blinded me to how maddening it can be to get a decent shield as a warrior or paladin tank. I still haven't gotten the Extinct Turtle Shell from archaeology, although it wouldn't matter for the draenei warrior on a different server from my main.
19. One of the things that consistently shocks people new to tanking is how much you have to know about dungeons and mob mechanics and how invisible this knowledge is until you run into someone who doesn't have it. On that note, Maps for Tanks is a godsend.
20. Heirlooms are great if all you want is to get an alt to 85 as quickly as possible, but one of the nice things about playing without them is being able to look forward to what drops.