We all know common sense isnt' that common, but I think that's primarily because of context. Everyone has common sense about something, and that common sense is only common to those who operate within the same context.

A designer's common sense comes from knowing his domain, from experience with that domain, and from experience designing for that domain. If you know what makes designs within that domain good, you can reproduce it.

Everything you need to know about anything can be learned by studying one thing completely. If you study a single rock thoroughly enough, you can eventually write an encyclopedia about geography. This is true because to study one thing completely, you have to study now only what's inside of it, but also what's outside of it. Far enough outside to understand how everthing else affects the thing you're studying.

Your common sense about something is derived from having lived within that domain for a while. Once you know the domain, because you've gotten really cozy with it for a while, you can step up and start morphing yourself into an expert.

That said, you should never stop learning as you become an expert designer, nor should you stop learning after you've established yourself as an expert. Everything changes, especially in the web business. No matter how good you are, the only way to stay that way is to keep up on your research, new trends, coding chops, and so on. You have to keep your finger on the pulse of the web at all times. You have to continue walking the walk, every day, day in and day out. Else you'll lose that "common sense" you worked so hard to gain.

Common sense isn't common at all. Most of us are lucky to ever understand something so well that it seems like common sense.