Ninjas and pirates! Yarr!
It's a polymorph effect. You change into either a ninja or a pirate. As with any other polymorph effect this prevents you from using your mount or flying. The recipe is a very rare drop (from any mob in the Barrens) and thus very tedious to grind for. Since the Delight is a favourite with many high-end guilds for 'fun events' it tends to sell well, again justifying a high price on the AH. The Deviate fish are easy to get so once you have the recipe you are set for a nice income as long as competition isn't too bad and you have the patience to fish.
PS Some people use Deviate Delight to preserve their dignity while naked. For example if you are stripping off all your clothes in Molten Core for a controlled wipe after Magmadar then a Deviate Delight will ensure you appear fully clothed despite having no equipment on.
Edited, Thu Aug 25 05:52:24 2005 by Morthandeus