To complete a to-do with a comment, the steps are multiple and require more clicks than necessary:
Add a comment in the comment thread for the to-do
Save the comment
Complete the to-do
This also kicks off 2 separate emails for the to-do completion.
Suggestion: Simplify the to-do completion so that you can complete it and add a comment at the same time.
Benefits: Only one email gets sent that indicates the to-do is complete AND provides the associated comment/