I also worked on SproutIt's Mailroom app, which is about 80% of the functionality of GMail, implemented by 2 dudes (one backend, one part-time front-end) over the course of a year (also in Ruby on Rails). Sprout's since released their toolkit, called SproutCore, but its docs are still a bit sketchy.
My $.02 -- too much of the plumbing for web 2.0 takes place on the server, which is why time to market on the server-side has been so important over the past few years. With Prototype, MooTools, YUI, etc. a lot of the fancy front-end tools can be bolted on after the fact.
Also -- how many apps fit the mold of a 100% full-screen AJAX app? Email, spreadsheet, word processing, calendar... what else? Reading news, surfing blogs (outside of an RSS reader), googling for information, etc. do not fit the mold of a 100% AJAX app.
Everything in GWT is treated as AJAX app, in that all RPC's are asynchronous. But all GWT apps dont have to be "100% full screen". You have the freedom to do whatever you want visually, including wrapping JS libraries for animation, drag drop, etc (which I've done with my current project).
That being said, I agree with your comment about not everything fits the mold of an Ajax app. Like if you are not building a web application. Rails will be faster and more immediately gratifying in most situations.
Now I DEFINITELY don't want to use GWT.
That is a bit odd, its a bit like saying you couldn't get Dojo to scale so you switched to Postgres. I assume if their startup was happy with ajax idioms in RoR, then they would be mad to build it in anything else, as it does so much for you (but if you stray too far, ouch).