labels
not only are they superior to folders (an email can have multiple labels, but can only be in one folder), but they make searching vastly easier.
Suppose I wanted to find an email I sent bob, and I know it as an attachment:
from:me to:bob has:attachment
POP3 and IMAP support
integrates with web-accessble, shareable calendar
integrates with google maps (automatically asks about locations in the email) ABOVE THE ADs!
spam in the inbox is unheard of
No GUI ads
integrated chat/video client
automatic emoticon interpretation
can be set to https-only