dream merchant has mentioned alot of great points.
here's my contribution.
somehow in the modelling world, there is no beautiful or not beautiful. where right and wrong does not exist on the photo.
what i feel are the prerequisites of a model are honesty, telling others that u have commitments before hand that may affect a planned shoot is crucial and can cause losses to many people involved.
punctuality. often, whether u are in a paid or free shoot, timing is important. u cannot simply waltz in hours late without prior warning or notice. even then it's no excuse. it simply reflects on upbringing, discipline and professionalism.
professionalism. sometimes when the client or photographer is an idiot, that doesnt allow u to throw tantrum on the scene or refuse to do things agreed before hand or terms in a signed contract. plus, leave yr personal problems outside of the shoot. u can pick it up later when things are done.
for now i can only thing of these. may add if i have more.