I absolutely loathe Y! Mail's new interface. It's suppose to resemble Outlook and Oddpost (Y! acquired them) folks have spent a lot of time copying it. Problem is that Outlook's UI is not that great to begin with. All that clicking and dragging and split-pane views are so damn annoying and slow.

More here: http://www.sitepen.com/blog/2006/12/18/why-splitpane-views-suck/

Gmail has Y! Mail beat even with this storage system.

I also don't trust Y! as much as I trust Google... that China incident just turned me off Y!.