The options for cooling cases is my personal muse. I've noticed how in the last 3 years watercooling has gone from a DIY project requiring many tools, pipes, and random car-parts, to something more-or-less modular, and companies respect that.
Passive cooling is something else altogether, while not yet on par with other methods, it is awesomely silent (for obvious reasons!). I hope more people come to realize the computer design is as much aesthetic as it is power-based.