In a similar vein to submersible_toaster, I advocate for doing everything possible within reason to smooth over the lack of real desktop presence. Highlights might include:

• a noticeable but not-too-intrusive indicator for new mail that pops up no matter what else I may be doing (browsing, composing, managing contacts etc)
• navigation by keyboard shortcuts
• more-or-less real time address book resolution
• more-or-less real time spell check (which can be disabled)
• drag-and-drop of attachments from OS GUI (would require some custom software to be installed on the client but could be very worthwhile for certain users)
• Provide, or at least simulate, a continuously scrollable message/thread list with no need for paging