Well, for one thing, classic Outlook's HTML rendering engine (to the extent it's even an "HTML" rendering engine instead of a Word document rendering engine) has some fairly radical divergences from standard HTML/CSS behavior. For those of us who have to generate HTML e-mail for whatever reason, it's distinctly unpleasant.

That might not be enough reason in and of itself to throw out the whole application, but I sure won't miss that one part of Outlook.