I'm a dedicated GMail user, but I think it's good to give credit where credit is due.
Autosave was definitely not first in GMail. Thunderbird also allowed messages to be sorted by thread a while back (which you could argue was a rough precursor to GMail's conversation view).
I really like Outlook. I switched to Gmail this year because I wanted free e-mail and Gmail is easier to configure. But, as a client, I enjoyed Outlook much more than I do Gmail.
Small nit, Outlook 2003 had AutoSave. GMail was publicly introduced in 2004.
> embracing the fact that it is not truly productive to file every single e-mail into a purposeful folder
That's the "Inbox"
> embracing that tagging is better than folders.
I'm pretty sure that Outlook has had categories (the color-coded custom named things that can be multiply applied to emails) since 2000ish.