it would be very useful to have a short comment field on the input spread sheet especially when modeling larger more complex systems or to leave notes to other analyst who may use the model

Yes, that would be nice. The only excisting feature somewhat in the same category that I am aware of, is the "Name" checkbox that allows the user to name the different nodes.

I think it's accomodated within the results with annotation features, however I agree that a comments box on input would be a useful addition, maybe with a summary of comments.

There is no element-based comment field (other than that label mentioned here) but there is a title page available for your notes. There's a button to this page in the static input window.