I'm currently using a setup where I can see incoming mail in a taskbar (unread count and subject), press Win-o to spawn mutt, enter to open first unread, z to archive, and qq<enter> to close mutt. With this, a lot of email is dealt with in six keystrokes and under two seconds total (no interruption in workflow).
I'm not saying that you wouldn't be able to do this with a web-based mail client, but I think that the machinery to do that would be less natural. (My sense of what is natural might be warped, though.)
Also, there is much to be said about the ability to customize your mail client to suit your needs. The very fact that people are complaining about Gmail's new design shows that a lot of people are relying on Google to tell them how they should manage their email, and try to adapt their needs to the tool rather than the other way around. This is not a technical limitation of web-based mail clients, but most webmails I know aren't really hackable.
> I'm not saying that you wouldn't be able to do this with a web-based mail client.
In fact, using gmail should take fewer strokes than that. Assuming you have a keyboard shortcut to spawn a gmail window, enter will take you to the first message, and 'e' will archive that message. Command-Q/Alt-F4 to quit.
Labels can be applied with an 'l' and the label name, next/last message is 'n'/'p'.
Go to Labs and enable Custom Keyboard Shortcuts, then go to town! I changed a bunch of my shortcuts, for example an operation I do quite often is "select all unread, them mark as read." But I couldn't remember the shortcuts they assigned - they didn't seem mnemonic at all. I changed them to SU for Select Unread and MR for Mark Read. Bingo!
> It would, of course, be better if they let you customize the shortcuts and sped up the loading of the initial page.
Those were the main things I had in mind (along with adequately customizable desktop notifications). As for shortcut customizations, the Labs feature isn't as good as a configuration file in my opinion (slower to edit, harder to share, harder to diff, etc.). As for the loading of the initial page, I still wonder why it takes so long... You could probably get a nice setup by keeping a Gmail window open and assigning a shortcut in your WM to display this window, but it's not a common feature in WMs.
I'm not sure it's universal, but I have "Custom keyboard shortcuts" available under Labs that let's me customize as desired. Click on the non-intuitive gear-shaped icon in the upper right corner to check if it's there for you too.
