--Before I started to write fanfic, I didn't realise how great the difference between academic English and story-English is; I needed a whole new vocabulary! --

Indeed! Same here. And I find it very difficult. I have those scenes in my head and lack the vocabulary for them. LOL. Dictionaries have become my best friends. But it does improve my English, I hope. Especially my written English. Other than with casual posts, with fanfic I have to pay attention since the archives I use are strict and I don't want to cause too much trouble to my betas. And I so suck at punctuation.
I'm quite good with my own language and it bugs me that I'm not as good with English. LOL.