Long-time Yahoo mail user here (it's actually hard for me to admit this; I've noticed in software/tech circles that Yahoo increasingly has a stigma attached to it, not unlike AOL in '90s.)
Anyway, one big difference I notice with Gmail and Yahoo mail is Google actively develops and improves the parts of the service that is visible to users. Every few months there is a significant update (recent ones that come to mind: G+ integration, new inbox design, some new templates, etc.).
Yahoo mail not only fails to add/integrate new services, it doesn't even bother with basic tweaks that it can see over on the competition (Gmail). For instance, Gmail has this very prominent and useful feature: "Always show images from xyz@blah.com". Yahoo mail? It's still on a message-by-message basis. It's irritating, but also kind of sad -- Yahoo has a popular service that's still used by millions of people, but doesn't seem to care. The same can be said about Flickr, too.
Yeah, I'll give GMail that. Yahoo's image handling does seem a little hit and miss - I've got some senders to always show, somehow, but it's not as good.
But every time I use GMail now I find myself using keyboard shortcuts from Yahoo and wondering why they don't work. The interface jusst seems ugly and crude. Filtering I don't find as good - sorry, I've tried their labelling system, I disagree it's better and the setup interface is poor. I've noticed the spam folder grabs a lot more false positives, too.
GMail is good, sure, but I odn't in any way regard it as a stand-out leader. Yahoo Mail may not be improving as fast as we'd like but it's frankly still a very good product.
Anyway, one big difference I notice with Gmail and Yahoo mail is Google actively develops and improves the parts of the service that is visible to users. Every few months there is a significant update (recent ones that come to mind: G+ integration, new inbox design, some new templates, etc.).
Yahoo mail not only fails to add/integrate new services, it doesn't even bother with basic tweaks that it can see over on the competition (Gmail). For instance, Gmail has this very prominent and useful feature: "Always show images from xyz@blah.com". Yahoo mail? It's still on a message-by-message basis. It's irritating, but also kind of sad -- Yahoo has a popular service that's still used by millions of people, but doesn't seem to care. The same can be said about Flickr, too.