To start I was making a few levels just to have something to play with. As I started building more I realized I needed them to flow better from one to another. On top of that, each one really needed a theme, puzzles, and better flow. That's really hard to do in small sittings. For me at least. Its brainstorm that requires a lot of rough drawing and planning that has hurt me in the long run having to go back and redo chunks of levels.
I have one level I actually planned out at home, drew stuff up on, and then built on my train ride. It's probably the most solid level I have. It made me realize that just building on the fly wasn't going to work. So now to really make levels I'm proud of I'm going to have to invest more time up front in planning.
Level design is about more than just the actual layout of the stage. It's also about why you go certain places, the story behind items, and making things designed in a way that makes sense practically as well as for game play.