1) Your first map will be crap. Hell likely your second, third, fourth, fifth...and so on if you're like me will be crap
In general your early works won't be good and for the love of god never release your very first playible map, if you have no experience mapping the playability is limited to the fact that people can spawn not any thought on gameplay.
2) Avoid CSG subtract at first. If cutting a square hole out of a square solid wall then go for it. However cutting a multi surface area at wierd angles leads to more headaches at compile time than building brushes from scratch. Get into the habit of building regardless and you'll never miss CSG subtract.
3) Research your vid card against how trem normally plays. ie. If on your card Tremor is super bright: It's just THAT card. Most everyone else has to turn gamma up to see in Tremor so making your maps 'dark' on yourcard means it's likely going to be pitch black (and thus unplayible) to anyone else.
4) Clip brushes are your friends. Learn to use them in the right spots.
5) Eye candy is nice....modelling the statue of liberty in the center of the most active route is a bad idea. Beautiful maps are nice but suck heavily if fps dies. Simple maps look boring but play like glass.
6) Gimmicks are a minority in priority. ie. Traps, special effects, wierd rooms, easter eggs, gags, disco rooms all should be a minimum and memorible rather than plentiful and down right annoying.
7) Lighting. Read the tutorials. Practice. Read again and ask questions. Lighting can make or break a map.
