I got a sneak peek of the new Yahoo! Mail client released tomorrow under limited beta. It's the first version driven by the Oddpost team acquired a year ago. You may recall those guys doing Ajax before it was cool.
On the upside, this is the first webmail client that doesn't feel like it's on the web. It has the look of Outlook (with Yahoo branding and an ad box), slick drag & drop across 3 panes, real keyboard shortcuts and right click menu, and wicked fast autocomplete and caching that lets you scan through folders with thousand of message even faster than Mail.app and other desktop clients. Some of it will be familiar to Oddpost users, but now it works across platforms (including Firefox for the Mac!) and with Yahoo hosting, storage, search and spam protection it's simply of a different grade. This will be one of the best examples of Ajax pixy dust others will seek to emulate.
On the downside, they haven't incorporated some of the features that could make webmail rule the earth. RSS in and out. Tags are the new folders. Social networks as filters. Synch to support mobility and our occasionally connected reality. It's also simply a different design philosophy from Gmail -- automation for the mass over augmentation for the next -- and only markets will decide.
Oddpost did have RSS aggregation as folders. I do believe that openness has sunk into the Yahoo! being. This does give them a leading interface platform they can integrate across the Yahoo! portfolio, build upon and let others build upon.