You can change the subject line of the email to be the value of one of the form fields that you collect with the form. But this only gets you a subject line that includes nothing other that the user's name (or which ever field you choose).
To do so, in the form processing steps, take a look at the configuration for the "Send Notification Email." Under "Choose how the subject of the email is selected" you can choose "Use the value of the form field to select the subject." See below:
To produce a smarter subject line you would need to use up a new contact field, a hidden field or two and either a Cloud Connector (String: Concatenate) or a little bit of JavaScript.
I recommend that you either stick with the original subject line or go with the bare-bones subject line that just has the user's name as the subject line. If you have a nice email filer setup in you email account this might be enough for you.