These are a few of the tricks I've found while transforming into different things, I've had a look around for these and found mentions, but nothing listing all of them. I realise this is a bit late as the events almost over, but *shrug*
I was going to post this in the exploits forum, but they're not 'exploits' so much, as this is what blizz meant them to do.
Skellie costume - Able to breath under water, good for farming in those hard to get to places, especially for motes of water.
Pirate Costume - Higher swimming speed under water.
Ghost costume - Able to float over water, which makes fishing easier in alot of places, also gives you featherfall so you can leap from high places. Possibly good for getting across the sea, I've not checked for the fatigue meter yet.
I hope this'll help a few people, as your posts have helped me tremendously.