On the mailbox side, Yahoo is offering what it calls a “smarter inbox”, a welcome page combined with a prioritization system. It also includes integration with multiple Yahoo services, as well as the ability to provide instant access to third-party services as well. The latter is probably the most important part, and where Yahoo expects to see the most feature-richness come from. They want to see users accessing things like Flickr accounts, blogs and more through their mail account. They're also including Yahoo Updates along with the new mail package.
On the toolbar side they've provided new email features, personalized searching and other functionality. The company, ailing as they are, has a massive user base with which to help them recover.