A special tip to Captain P:Make a grunt with an unavoidable GL. sarcastic -
Never forget to ask yourself: is this
fun to play?
Some things aren't impossible for a player, but they aren't fun either. It's hard to strike a good balance, but it is important. That's also where (multiple) playtesters come in so handy. I've got a small Dutch forum and that's always a nice group to get feedback from. Taking feedback, whether you like it or not, is an important thing.
Again, don't abuse your freedom. For example, in HL2 you can create much larger maps - but without vehicles, and with the current slow movement, larger maps are probably not the wisest thing to do. When I was testing a layout, I quickly found it becoming very boring because a lot of paths were just too long. Someone from the UT community once said that paths between area's or intersections shouldn't take more than 2 seconds to travel. In a multiplayer game, a player wants combat, not exploring the same (long) corridor over and over again.
So, if you can do something, consider if it's an enhancement or not first.