I'd love to see CC implement an easier way to create segments, particularly from lists. At the moment, I have my contacts segmented into nearly 120 lists, and that number continues to grow. (I'm with a school, so we have alumni organized by discipline, year, etc., we have donors organized by event, relationship, etc.) Creating segments currently involves setting criteria to list membership, then selecting the list, then repeating the process again and again and again. It's more trouble than it's worth.
Would it be possible to have the option to save a mailing list as a mailing is going out? So, as you click send on an email, CC might ask, "Would you like to save the current mailing list as a segment?" That would seem easy to implement, improve the user experience, and get more customers to engage with the segments feature.