The one that came to mind today is Show don't Tell.
First of all, here is an example of telling (it's from a fic I'm writing at the moment):
Tom was out of the car so fast that no one could keep up with him. When the call had come that Bill had been found and was still alive he had almost passed out as the knot inside him had released. When he had demanded that Saki turn the van around and take him back to the hotel almost three weeks ago, the terror had started. Finding Tobi unconscious, Bill gone and Bill's room a mess had been the worst moment of his life. It had been the most terrible feeling and he had lived with that dread every moment of every day since.
Now telling is not always bad; sometimes you have to use it in places or your fic would be three hundred pages long before you made it to the point, but the above example could be so much better. The reader is only seeing the events in summary and they are very important events. What I have done now is ripped out that paragraph and started writing a prologue with the events being shown, rather than told. It's more work, but it will give the reader first hand knowledge of the setup to the story and hopefully give them more insight into the characters' states of mind. This is the beginning of it:
Sitting in the van watching the city go by, Tom had hoped that he could put his worry to the back of his mind. Bill was back at the hotel, lying in the dark with a cold pack on his forehead because he had a migraine. It wasn't anything stupendously odd; lots of people had migraines, but Bill had never had one before and it had been so sudden that Tom couldn't help worrying.
Now even this is going to have some telling in it, but the important events are all going to be there in black and white.
Showing takes more effort; you actually have to write what happens long hand, but it often pays off so well. Professional writers are as guilty of it as fanfic writers, so don't think we are the only guilty ones :).
I think one of the worst telling, not showing incidents I can think of is in Lord Of The Rings: Tolkien gives us this huge build up to the battle with the ghosts and Aragorn to take the ships on the river (the details escape me now), and then the whole thing is reported in two pages when someone is telling Frodo and Sam about it after they've been rescued. I felt incredibly let down when I read that; there was all the build up and then, fizzle; it was gone. Some will undoubtedly disagreed with me, but as I mentioned above it is just MHO :).
Sometimes showing can seem like such a drag, but what I do is walk away from the scene, go and write something else and then come back to it when I am in the mood for it. Sometimes I come back and think, 'no, that'll do, it's not that important' and sometimes (like above) I come back and think, 'don't be a twit; write the damn scene'