The "killer feature" for me on Gmail is conversation view, where it groups messages together in conversations, so instead of a ton of disparate emails, they're grouped together in a single line and can be seen in sequential order. Back when I switched over to Gmail, it was the only thing that had this feature, and now I find it indispensable, though it does sometimes screw up (since email was never designed to actually have this in the first place). Do other clients have this yet?
Yes: https://addons.mozilla.org/en-US/thunderbird/addon/gmail-conversation-view/. My experience has been that webmail is inferior to having a mail client. Even simple things like correctly displaying email which contains styled HTML content doesn't work in, for example, Gmail.