The only caveat I would add to this is that messages in the junk mail folder should probably be left plain - to slow down malicious links getting through that aren't otherwise caught -- 

(essentially mimicking Outlook behavior without necessarily saying it)

If I remember correctly, we do have a task for that: making links in text-only emails in your Inbox clickable. I know that when you view Archived messages that are text-only, the links are clickable. The difference is that Archiving uses a different viewer that's not, technically, a webmail client like the Email area is.