My biggest point of pain with backbone is the models, in one project I had to add lazy loading (pagination) and in another a sparse array implementation which was a lot of hassle.
The other point is boilerplate - there's a lot of it. projects like backbone-forms (https://github.com/powmedia/backbone-forms) try to fix it, but it's still a problem.
Pagination shouldn't be too big of a deal -- that's precisely the sort of thing that Collections can make easy ... ditto for modeling a sparse array (although I'm fuzzy on why you'd need that -- if it's for performance only, you should be using something more low level)...
As for boilerplate, I'm not sure what Backbone-forms has to do with it -- if you're suggesting that Backbone should include UI widgets for auto-generating forms ... I think that's very far away from the role that Backbone is supposed to serve.
I don't like feature request issues, once we are done with the current project we'll send a couple of pull requests.