To force users to make a selection of an attribute, I am using the commonly suggested technique of making the first attribute say something like 'Please Select', and setting that option to be the default and display only. If the user fails to make a selection, he is returned to the product page with an appropriate error message displayed in a yellow box. The problem is that if the user has enter a quantity, that quantity gets reset to its default value when the attribute error occurs. The user should not be required to re-enter data in a field unrelated to the error.
This problem can be demonstrated in the Zen Cart demo shop (ver 1.3.8a) for the product called 'A Mixed ON Product with Attributes'. The initial quantity is 4. If you change the quantity to 40 and click add to cart without selecting the Size, the quantity gets reset back to 4.