It's a pain maintaining override scripts for multiple sites, but pick your battles and you can make some positive improvements.
[1] at 55 seconds in the video. This link starts a couple seconds before that: http://www.youtube.com/watch?v=uDI6Itn7soQ#t=53
There's a ton of ways you can specify your own CSS for a website, depending on what browser you're using.
Not only is it getting harder and harder to read emails on a Netbook screen (there is almost no space left for the actual message), it is also getting painfully slow.
Gmail now loads and runs to much Javascript that my poor little Netbook can't even scroll smoothly anymore.
After playing with Outlook.com a bit, it seems much faster than Gmail.
Since Marissa Mayer is now at Yahoo, I expect a much improved Y!Mail soon.
Depending on where you are, the options on the toolbar change, and are really helpful.
I really like it.
Only one thing: Microsoft, I know its really chiq and en-vogue nowadays to have some element with position:fixed; on every page, but please: don't!
There is no reason whatsoever why I would want to see all the time the titlebar. I mean, why would I want to see it? I know I am using Outlook! I know who I am! And it's not very often that I want to access the settings!
But, I guess, there is no way around it, position:fixed; is just soooo "modern".