This is a big one, but wouldn't it be nice to have the kind of functionality that allows you to associate contacts with specific business opportunities and trace them through the sales cycle? Outlook doesn't really do this very well either, but it is a huge deal for some folks, like me...
We tightly integrate with Salesforce via our Zimlet.
Something we've been kicking aroun is Sugar CRM integration. . but we've been waay busy. . .so that won't be for a while.
We STRONLY encourage our community to try and put something together. You know what you want ;)