What is Your definition of successful web mail?

I'm reading a lot of opinions about Facebook's rumored "Gmail killer". I'm curious about the specific characteristics of any web mail that HNers would consider valuable? I am NOT referring to the solution of setting up your own domain mail server and linking it to browser interface or mobile client. I can think of two: (1) Easy download/backup: API for fast batch import/export, zip files via browser, etc. (something beyond IMAP). I would be willing to pay extra for this service; (2) Providing reasonable assurance that data associated with my account or with individual emails do not persist after the account or the emails are deleted. I admit the second characteristic is theoretically impossible without complete control and audit of the hardware. I hope someone can propose a novel transparent process (such as data being stored in escrow or audited by certified third party or something). What specific characteristics would you look for?