Real life can be very inspiring, ministeve...
Plus, real life can guide you to make your maps resembling reality better, thus creating a stronger sense of place... never underestimate reality...
Half-Life mods can be very inspiring, but hey, it's still not all that can be done with the engine! Every now and then someone finds a way to go beyond the limitationg by some trickery... Don't limit yourself!
Then again, don't take that as an excuse to ignore the given limitations... performance is important. Good-looking maps with 1 fps aren't popular, obviously...