Decepticins have these characteristics:
- They enjoy destruction/don't care about the consequences of their actions, and
- They have cooler vehicle (and often even robot) modes than the Autobots
I am not making this up. Soundwave - way cooler than Blaster. Devastator - way cooler than Ultra-Magnus. Even Starscream - much as I hate to admit this considering what an annoying character he is - is cooler than Blurr. And Unicron just beats all.
This said, there are a few pics up on Flicker of Decepticon vehicle modes. Check them out here.