I'll have to be honest, I don't share the same sentiment as most people I know who've been to Khao San. I just don't feel as wow-ed and I've been here 3-4 times so I definitely gave it a chance.
First, there's always a long line up and the seats are too close to each other. I sat in the communal table once and it was quite uncomfortable as we were sitting elbow to elbow with other people and could not have a proper conversation.
Food was good, flavour was there, definitely more authentic than most Thai spots downtown. I just don't remember it to be remarkable. I don't find myself wanting to come back, or craving certain dishes. I tried their bangkok style pad thai, pad gra prow, green curry and gra bong. All pretty solid dishes, so I think you just have to try it for yourself and see if Khao San will tickle your palate.