Meteor shows promise, but I have to agree that without a mature UI toolkit, you just end up with the same junk we've had for years (not speaking to other components of Meteor).
For what it's worth, have another look at promises. They're not the same as "callback hell" and can be a really elegant way to compose and deal with async. It's actually really nice, when the tool is called for.
Agree with you that Meteor's built-in reactivity is really nice!
I'm not sure why you're claiming Backbone isn't an MVC framework. It was pretty much designed to be MVC or at least MVVM, and checking the Backbone website, it still says it pretty much in the first paragraph: models, collections, and views. That's fine, but I like Meteor because it doesn't need the MVC crutch to provide structure and elegance. Somehow, it gives me more power as a developer without also asking me to write the kind of boilerplate code normally associated with data binding, getter/setters, models, and things like event emitters.
If you still prefer Backbone, that's great! But I just wanted to explain to the OP that for me, Meteor is about getting rid of spaghetti code, not adding to it.
For web apps, I tend to define spaghetti code as the huge layers of glue and boilerplate you have to write to get your data out of your database in the server, serialized into some JSON or XML format through your controller in order to send to the client and into an XHR callback hell or confusing promise-driven architecture, all so that you can finally concatenate everything into a string inside some jQuery function where you then have to manage your own state.
None of which even comes up when building with Meteor, since it takes care of all of that for you, leaving you more time to build your app. So as far as I've experienced building apps in Meteor for two years, spaghetti code is a thing of the past. Not to mention that just the fact that the API works this way dramatically simplifies your application architecture just as a side effect of how the templating system, reactivity and javascript-everywhere approach work.
Yes, there are MVC frameworks like Angular and Backbone that help you take care of the client side of that. But they don't handle the server at all, so IMO they still haven't matched Meteor's feature set.