Generally speaking, it takes insight to create good maps.
Mister Plod, what do you think make your maps different from Valve's maps? Can you pinpoint a certain thing, or certain things? If you can find out your maps weaknesses, then you're half the way up on improving them.
For ecample, my weakest point is creating layouts. I usually started detailing some rooms and adding rooms and all that, untill I got stuck at a point that I couldn't get a good flow into it. So, I started reading articles about it, looking at other maps and all, and changing the way I'm working. Now, I've got a decent layout ready for a HL2DM map and am busy doing the looks of it. Now, I want to be sure that it plays well before I start detailing it even further.