Hello Claudia (your nick-name and the picture make me think you have either German ancestry or actually are German)
Here in Germany it is pretty hard to find the yarn the patterns ask for and I just cannot always afford to pay a huge yarn-bill. So I try to adjust the patterns to the yarn that is available/affordable for me.
You both are right, it would be nice to have some kindof suggestions what kind of yarn we should be looking for. I mean, does the yarn used for the pattern need to be very stretchy, and so on. I sometimes just have no real idea about drape etc.
(I love knitting but to think about drape gives me the creeps.)
Cheers, Julia