AJAX isn't why Gmail became successful. What set Gmail apart from its competitors was the huge amount of initial storage you got (1 GB for free at a time when Hotmail offered less than 10 MB for free), a higher maximum attachment size (25 MB compared to the webmail standard 10 MB), Google search, and conversation view. All of which, by the way, worked with the AJAX-free basic HTML view.

Well, I doubt either of us have statistics for what really drove its success (and if you do, I'll gladly cede the point), but anecdotally, I and a lot of my friends at the time switched to Gmail not because we needed more storage (I don't remember even thinking about that as a problem with Hotmail at all) but because the interface was way better. If anything, I think it had more to do with the "cool" factor of getting an invite and the cachet of Google at the time than with the storage. But I freely admit this just personal experience talking.

Also the spam filtering was years ahead of it's time

I had forgotten how big a change conversation view was compared to other webmail.

Also for what it's worth, I still use the Basic HTML view because I find it a better experience, so AJAX was definitely not the selling point for me. The 1GB thing was a huge factor though.

For me, the basic view has always felt neglected and kind of halfassed. The full experience has fully styled UI elements where as the basic view just defaults to the browser defaults. Outlook does this too.

To me it just feels like basic view is just v1.0 from 2004.

It's definitely the uglier of the two views, no question. I prefer it functionality-wise though. It does less of its own window management, etc.