I think you're right about the built-in drawbacks of fanfic -- the way it can exclude readers who don't follow the original source, although I think the problem is worse in short fiction. I can often read a long story in a fandom I'm not very familiar with, because the plot of the novel itself will carry me along. Whereas for short fiction, I need to know the source (and care about it) much more deeply.

I'm pleased that you liked the writing--that it could pull you in even without any knowledge of the source. That's a great compliment.