It would be great if you could flag or label emails.
If you communicate a lot with your customer, it is really hard to find the important information in the deal history, especially if you search in deals from colleagues. I can find my own mail in GMail, but if I don't have access to the mailbox, it's almost impossible to find the right information. With a label or a flag you could mark the important mails and in a perfect world you could filter them.
As a workaround you could of course put the important passages as a note, but the other way would be better.