All meeting invitations should have a different icon (a meeting icon) for me to
distinguish between an appointment received and a mail received.   In fact, the
theory need to be extended to tasks too.   Today, I need to open the received
invitation mail to see the Accept, Decline...buttons to understand that I have
recieved a meeting invitation/appointment.

It is also better to change the font of the invitation summary or change it to
italics for better look and feel.

Other information:
Almost all the commercial email clients that I have used (GroupWise, Outlook,
notes) provide this.   Evolution should have this.