I lived in Bangkok for four years and know Thai food as well as anyone. Khao San Road is *absolutely* authentic Thai in every way and I've been looking for a place as good as this for 10 years. The wait staff are all mostly younger Thais and have that endearing and unique Thai friendliness and hospitality. The seating is very basic and communal, which contributes to the sense of Thai community (as well as gives the owner higher density.)
They've managed to find all the little Thai ingredients that give Thai food in Thailand its special appeal, and that's the taste I've been unable to find. I think it's a smart idea to have a very limited menu because the chefs are able to turn around the food quickly, and there's enough on there to keep each visit a little different. I can't believe they had Khao Soi from the Isaan province in northern Thailand--I've only ever found that in Thailand (and it's wonderful.)