Well, you can achieve what you're asking for without any loops in SPD workflow. Think about how you want to trigger this action.
Let's say users can manually trigger such a notification email. Give them a form (i.e. InfoPath) that queries your list and pulls in the data rows you want to include in your email notification. Set a rule in your form to concat the returned items into a text field. Your Submit button sends the result to a library on which you trigger a simple notification email workflow - in the body you put your text field. You can always have the workflow delete the item after the email has been sent.
There are other options - you could for instance create a page based on parameters with a data view web part setup to filter by whatever you want to include in your summary. Then simply add a link to that page to your notification email and ask the user to access the information that way.
Hope this helps a bit.