[Koji]

Metroid games have a lot of lessons to teach; examining them with a critical eye (as well as any other game) would help you a lot. The original NES Metroid, for instance, starts by laying down its design principles: You appear in the middle of a 'cave,' with alien architecture and two hostile-looking creatures (they have spikes) near you. First reaction is to shoot them down (as you should soon learn that you have the ability to do that and jump,) or simply avoid them. But then you have the choice to either go left or right. If you decide to go right you'll face some more monsters, and eventually reach a dead-end: there's a small tunnel half your size, but you can't crouch into it. So you go back left, and find a structure that has the same small tunnel underneath, but this one you can climb. So you climb it and reach the other side. As a player you might or might not notice that you can't go back by those same means, but you're trapped in a room that you can't exit until you grab the Morph Ball item, which sits on a prominent figure, so you can't miss it. You grab it and a fanfare plays: you know that you did something right. So now you might notice that you can't go forward or back by simply jumping or blasting, so you attempt something different, and eventually learn that you can now roll into a ball and go through both tunnels you noticed before.

By this short sequence you have learned that the game is about exploring, that there are hostile creatures in the area, and that you should find items that will help you overcome different obstacles.

Don't create just obstacle courses, put meaning into your level design, in every sense. That's the best advice I can give you.