Bit of a storm in a teacup if you ask me. The offending tweet that made him think of leaving was something along the lines of "Stephen Fry is very clever and amazing, but his tweets are a bit boring".
Mr Fry says he's feeling a bit low; I think that's why he's oversensitive right now. Twitter is like anywhere else on teh intertubez- 50% interesting/nice people, 30% trolls, 20% adverts. I think I may be being generous there.