So I've been working in several email systems comparing and contrasting the features. Here's my take:
Yahoo! Mail: I have a Yahoo! id, but did not set up an email account originally. I tried to sign up to test things out for this blog entry, but I get an error message. I dutifully reported the issue to Yahoo support (issue # KMM58274977V28957L0KM), but have not had any updates in the past month. I don't think I should be forced to get a new Yahoo! ID, so they lose.
GMail: Technically a beta product, it's pretty open to people. If anyone wants a GMail ID, let me know. I have most of my lerch.org forwarded here. I don't use the mail search all that often, but I like how the filters use the standard search terms. The filters are very flexible, and it's nice to be able to tag items with multiple tags (basically the equivalent of putting a message into multiple folders simultaneously). It's very "web-like", and the ecosystem is also nice. GMail notifier, browser plugins, the ability to see mail on Google.com's personalized home page and POP access all make the system very accessible. On the downside, visiting GMail with two browsers simultaneously really slows the system down.
Windows Live Mail (aka Kahuna): I'm now using "M2" of the beta product for all my commercial mail from lerch.org. The idea was originally that it would be the account that would collect spam, but it turned into a bit more than that. Anyway, it's basically a trimmed-down version of OWA (Outlook Web Access), with an Outlook look and feel. If you like that, you'll like this, and you can get automatic notifications through MS Messenger. Also, rules that you define have priorities, so if two rules match the same message, you can define which one should apply.
While Windows Live Mail is very much like Outlook, which should be comfortable for a lot of people. It does not have POP access, and it doesn't have the ability to keep a copy of a message in multiple folders. Also, it does not support all browsers (connecting with Firefox downlevels to a Hotmail interface, although as a rapidly evolving beta product Windows Live Mail may address this issue shortly).
Right now, I do like both, Live Mail primarily for the very polished interface, GMail for the ecosystem support and filter behavior. I seem to enjoy GMail better, and it's nice to have POP so I can truly archive my mail if I ever want to move away from the system. I look forward to both systems moving out of beta.
Now, what I find really interesting is Windows Live Domains and the fast follower GMail BYOD (Bring Your Own Domain) service. More to come on this...