What sets Dojo apart from most other Ajax toolkit is the maturity and control, as well as amount of features.
Dojo has gone through several major redesigns in the last years, resulting in a very logical and organized API. Namespacing is one key feature here.
Dojo is one of the few frameworks which organizes most of its components or widgets. The normal thing to do when creating or extending a widget in Dojo, is to submit it for contribution, where many otehr frameworks have a lot of similar widgets spread out on individuals websites. This process also gives a minimum of sanitation to inclusion and rsult in Dojo shiping with a large number of integrated features.
Dojo can be used as a classical Ajax framework, for just doing CSS queries on an existing page and modify markup and event handlers, but the place where I think Dojo really shine is when you create real applications with it, and leverage the data stores, message buses and facilities for creating custom, reusable widgets.