Steve Trefethen's posting about Yahoo is pretty dead on. I used to use the various Yahoo apps and services, but I have slowly replaced them with the various services from Google. I think the problem was the development team saw too many bright and shiny objects and decided to implement them.
Yahoo's email is an over-developed mess. It's trying to be the web version of Outlook. Even Microsoft's web version of Outlook isn't trying to be a clone of the desktop version of Outlook. With web-based email, I want to get in and read it in as simple a manner as possible. Thanks to the GMail Manager plugin, I have notifications when email comes into my various gmail and Google Apps email accounts get mail.
I don't want yet another proprietary IM client. There's nothing wrong with Yahoo's IM, it's just that no one that I IM with uses it. I prefer to use Trillian, one client to serve them all.
It's kind of sad. I remember 10, 12 years ago when Yahoo was the only game in town for searching. It got slower and slower to use and when Altavista came out, I dropped Yahoo for speed and simplicity of a fast search app. And I stayed with Altavista until Google came out of nowhere and changed the search engine game as we know it.