How on Earth is Yahoo! Mail the webmail client with the best interface?
This drives me nuts. Gmail is the most functional webmail client, Outlook.com is the most attractive and has great integration and extra functionality (like seamlessly populating your contacts from your linked accounts, for instance), but Yahoo! Mail is the only one with a really good user interface.
And it all comes down to offering tabbed messaging. Why does no one else get this? Yahoo has had it for years now. Gmail is clunky enough that it's easy to right-click and open messages in a new tab in your browser, but it's kludgey, and relies on auto-save to populate it to your other Gmail tabs. Outlook.com doesn't appear to have any way of doing it at all, since it takes over right-click functionality and uses it for mail functions. (I don't use Outlook.com very much)
But really, it's such a simple idea. I can't be the only one in the world that needs to write a new message and then refers to other e-mails that have been sent to me, sometimes even doing the unspeakable and copying and pasting between them. Yahoo mail makes this incredibly easy, transparent, and functional. And it means that if I have different e-mails in different folders (or archived), they're open until I choose to close them, and I don't need to hunt them down again.
Now get on with it, Microsoft and Google!