Transmoggery

I love transmogrification in fact I think I'm actually addicted to it, there I said it!

Seriously, this feature is simply awesome,it has given me the greatest satisfaction in the game outside of raiding since I became unable to level my Shadowsong army.

The thing I like is that it allows me to have my characters looking good regardless of their gear sets. On Evlyxx that wasn't overly hard because a few weeks into a raid tier she had many items that even if not a set were part of that tier of raiding's theme. But my alts usually looked like they were wearing any old thing that came to hand (and to be fair they were!), however, now it is a completely different story!

Apart from looking good, it also allows me to remember what gear set they are wearing, a good example is of my paladin, Eusebian. I have 3 gear sets (1 for each spec) previously there was little visually to identify the set I had on and on more than 1 occasion I had gone into a fight as a holy geared tank or worse a tank geared healer. Now it is very obvious which gear set I'm wearing.

Eusebian in holy gear (left Judgement armour), tank (centre, Judgement recolour) and retribution (right, Wrath recolour)

There is more to the whole transmogrification scene than simply looking great. Firstly, there is the revitalisation of old content. The armour sets above needed to be collected from Molten Core, Blackwing Lair (Judgement armour) various Burning Crusade heroic and normal dungeons (Judgement recolour and Wrath recolour). Molten Core can be solo'd by most classes easily (some bosses are harder than others for certain classes) and Blackwing Lair needs 2/3 players for some bosses and the Outland Heroics are easily soloable by all classes. While this is good for everyone there are also some great side effects for you the humble player. 

From the vanilla raid instances, you can expect to grab a couple of hundred gold per run, in the Burning Crusade dungeons the average drop in coin and vendor trash is 70g (as Vile documented in his 2011, 3 part Outland Dungeon Runner Farmer 1/2/3), which whilst not earth shattering isn't bad for 20 minutes of work in trying to get your drops. 

There are also tons of greens, some of which are also potential transmog items so you really should check the way these look before blindly getting them disenchanted for the raw materials. While you can use the good old fashioned CTRL-right click a much more elegant solution is to use the excellent add on MogIt.

In addition to the greens are cloth and rep mats (Fel Armament, Arcane Tome, Mark Of Sargeras, Sunfury Signet & Coilfang Armaments) which can be sold, processed or used to increase your faction standing with the relevant Burning Crusade factions). Another trick for rep grinding is to equip the Horde/Alliance city faction tabards (sadly the Wrath and Cata factions don't get rep from the Outland dungeons).

So all told, you can make your toon look bad ass, have a ton of fun (especially if you take a friend) and make a crap ton of gold.