How can I make a weather-proof bike? Suppose want to park my bike outdoors all the time. Conventional wisdom is that you should park a bike under cover, but suppose that's not an option. What should you do?

Maybe sealed cartridge bearings would be better than cone-and-cup? 

Maybe belt- or shaft-drive, or at least a full chain guard to keep rain off the chain?

Maybe a treatment to protect the tire rubber?
Steel frames are strong and durable, but can rust; aluminum would be a better option. (Carbon fiber or titanium won't rust, but these are expensive enough not to want a bike left outside for security's sake.) 
An internal hub is sealed and will eliminate most drivetrain problems, especially if you don't use a front derailer. In addition, cleaning an internal-hub-only drivetrain is faster than on a traditional dual-derailer setup. Downsides: A bit more expensive, and removing the rear wheel to change a flat is a little more complex. 
A chain guard isn't a bad idea, but a lot of the gunk that gets caught in the drivetrain is kicked up by the tires. Full fenders will go a long way towards keeping your drivetrain (and you) clean. 
As icc97 has recommended, a stainless steel chain might be helpful, but replacing a chain is very inexpensive if you do it regularly. 
Anything made of leather will not wear well (mostly saddles, grips, and bags) so you would do best to avoid it on an outdoors-dedicated steed. 
Attachment hardware, such as screws holding stuff like racks, fenders, shifters, brake levers and so on will rust. It's a good idea to keep an eye on this, cleaning or replacing as needed. Screw threads will sometimes also rust, so keep an eye on that. Also, frequent cleaning in general will help enormously. 


Much of the above can get expensive. If you don't ride far or fast, there's the opton of finding a cheap, used bike and replacing it at the end of the season; used bikes can be found fairly easily if you're not picky.