An ability to take the reader out of their computer chair, out of their living room, and have them standing there in the story with the characters. I've been reading (slowly) 'Master Plan', and I keep having to remind myself I'm not on board the Valiant, standing just behind the Master's shoulder.

There's also the tendency for your stories to point something out to me that is (and this is totally a compliment) so very true to everything I get from canon. I tend to nod and say 'oh, absolutely' when reading your fic.