Ah, my first map... ctf_boats. For starters, a silly thing I did as a beginner was to plan the map's entire gameplay to revolve around getting on a boat to get to the enemy base. I even made the water poisonous below the surface (roughly) so you couldn't cheat the system.
Beyond that, I made the entire map using a grid of 1 unit, and I had no idea how compiling worked, so I paid no attention to leaks and such. When it came to detailing, I never got too far, but I did try to make some stairs, which went horrible due to my lack of a grid. Plus, I didn't know what func_detail was. Oh yeah, let's not forget how I used only prop_dynamics for some reason.
For my next map, I got a lot better, I just sucked at layout, scale, detailing, etc., but fortunately I learned from all those many, horrible mistakes.