I haven't read about the matchmaking recently, but I thought the idea was to match you up with people roughly your skill level, making a 50% win ratio the norm. That said, the keys to Starcraft are easy, it's just the implementation that's difficult. From what I've seen, the different skill levels are as follows.

Decent Player

Decent players separate themselves from n00bs by managing their economy. Like Fantatic-Templar was saying, always be building, expanding, and spending. If your money piles up, you aren't producing as much as your opponent is, period.

Good player

Good players know build orders and counters. Easily memorizable "optimal paths" exist to any particular tech or strategy, and are on the web for you to find at your leisure. As far as counters, good players will always be scouting the enemy, and adjusting their tactics accordingly. If you see an opponent going for unit_x, what should you do to stop him? How can you take advantage? Again, this is something that's fairly easy to memorize or pick up as you go along.

Great players

Micromanagement. Great players are all about the micro, and can micro without messing up their economies, build orders, scouting/countering, etc. Search for videos of high level players, and this is what will immediately stick out. Not only are they constantly producing armies, but armies are always having quick skirmishes, performing feints, and so on.

Anyway, that's about it. There's no real silver bullet to Starcraft. Know the best ways to build and maintain a solid economy, adjust to your opponent's tactics, and micro well enough to do better than a blind attack move. That's it. The challenge is just getting GOOD at it.