I don't know if they've changed it in the more recent films, but in the early ones the uniforms were grey, not black. Particularly noticeable in the scene in Philosopher's Stone where Ron upsets Hermione after charms. In the books, they wear black robes.
As someone who wore a school uniform, I believe they should be as uncomfortable and embarrassing as possible. It gives it a more genuine feel for me. At my school we girls weren't even allowed to wear trousers until we petitioned the school about having sexist policies.
I'm very for the shapeless tunic sort of uniform for girls and shorts or trousers & shirt for boys. Also, most of the public schools where I live have different uniforms for juniors and seniors. So you'd have tunics for the junior girls and a shirt and pleated skirt for the senior girls, and the senior boys would have to wear a tie or different coloured shirt or something (though some private schools do very interesting things with blazers and boat hats, too). Besides, if you've ever worn a robe, you'll know you have to wear something under it.
I might post a drawing of a hogwarts uniform, if I can be bothered actually drawing it.