I use the Gmail web client with rich-text and often find that my emails look to be formatted consistently when I send them, however when Outlook users reply, the text in my original email often has different formatting from line to line - sometimes with different colours and usually with different fonts.
I then wonder if the Outlook users are receiving my original email with inconsistent formatting, or if Outlook is doing something to break the formatting when it creates the reply.
I guess it's possible that this could be a labs related issue, but I haven't tested it very much. It does seem to be only an issue when emailing with Outlook users.
I've set my default text styling according to this Google Lab feature: http://gmailblog.blogspot.com/2009/12/new-in-labs-default-text-styling.html
Curious if this is a known issue/bug between how Gmail handles formatting and how Outlook handles formatting: http://www.google.com/support/forum/p/gmail/thread?tid=7b468e78dd0771b4&hl=en