The first time I fell flat on my face while levelling as a tank (which was also the first character I had levelled into Cataclysm content) was the second boss of Blackrock Caverns, where you have to drag him through the lava to remove his buff. Up until then every experience I had had had taught me to just burn through and hope for the best, and never ever show any signs that you might not be 100% up to date on the instance. Unfortunately with the way the game is currently designed that works very well up to and including most of Wrath of the Lich King, but then it comes back to bite you in the rear in Cataclysm.
Anyway, if I'm allowed to change the subject for a moment... transmog gear
Set #1 is what I'm currently wearing for tanking. It's basically the paladin T4 set with matching belt and boots from Karazhan, plus a random green mace and shield that happened to fit very well with it. I like the uniform look, although it doesn't look very impressive.
Which is why I'm farming set #2, from the Icecrown Citadel 5 man instances. I'm about halfway through completing the set, with the added bonus that the 1h sword that drops from that place matches the set too, meaning I have a transmog weapon in case I get my hands on Souldrinker.
Set #3 is my current DPS gear. Season 4 paladin arena gear with a green 2h axe from Outland that I think looks great, even if the gold is a little too bright on the axe blade. The 2h sword that you get as a reward from completing the new Redridge quest line has a similar dark blue/gold grip, but it looks a little too plain. This is why, should I be lucky enough to get a Gurthalak, I'll probably be using...
Set #4, which is basically the HoT tanking plate and the season 10 2h sword. While they don't match up perfectly, I think the combination is still very nice. I also like the details on the plate, especially the glow on the dragons' mouths in the shoulders.
Anyway, all this rambling about transmog sets made me realize that the game probably should have some sort of 'Fashion Victim' achievement for anyone who spends more than 1k gold on transmogrification.
Also, Madness achievement brag incoming in ~1.5 hours.