Taking cue from some other webmail platforms I've been using for several years, Zoho desperately needs one small feature which would improve the experience multifold and won't take any time for the developers to implement.

Zoho should autosave every email id in the To: and From: fields from both sent and received emails to the contacts. A future extension of this can be, to reference all such email addresses to google+ and Facebook contacts of the Zoho user to add name and number instead of just the email id.

The other suggestion is email threads. Almost all webmail have it. Its not a big need, but I think users would rather have it than not.

Hope this helps Zoho become better!