One annoying thing about Outlook is that it will show emails with graphical signature line as having an attachment. Since having attachment sometimes is an important filtering criterion for me to search for important attached documents, this becomes a pain. I ended up hating all the people who have fancy graphical signature lines.
What I hate instead is that Outlook is not paying attention to many small usability issues. When I tested this with Yahoo mail and Gmail, both of them correctly identified inline images as not attachments.