Some of Yahoo Mail's new features include a number that were previously only available on premium Mail Plus, such as disposable email addresses, up to 200 filters, automatic email forwarding, offline access and photo-rich themes. While Gmail has been using themes since 2008, Yahoo Mail's service has included high-definition backgrounds and the option to use custom photos, which are both applied across devices.
While it may feel like Yahoo Mail is taking the easy way out mimicking some of Gmail's attributes, Marissa Mayer may have the jist of it - don't mess with something that isn't broken! If Yahoo Mail could not figure out a way to out-innovate Google, why not try and add onto something that they have established and is working? If you can't beat 'em, join 'em!
In any case, Mayer and Yahoo! cannot rest on Google's laurels - they will have to figure out of way to legitimately compete with Google, but as each day passes it seems like Mayer's job is getting harder and harder.