Dojo provides a complete collection of user interface controls, giving you the power to create web applications that are highly optimized for usability, performance, internationalization and accessibility. Dojo is heavily based on MVC principles and provides powerful helpers for MVC-structured applications. It also provides widget infrastructure (with attention to details like – support for i18n and i10n), asynchronous loading of modules (if desired), integration with CDNs (content delivery networks) for heavy-duty web applications and so on.
Personally, I think Dojo's lack of good documentation/reference code, lack of 'marketing' and relative complexity keep it from becoming popular. Documentation is a whole lot harder to use than jQuery’s because, at first glance, it’s quite a bit more scattered and substantially more API-based than task-based. It's much easier to get started with library like jQuery because of all these factors. However, once you get over that initial steep learning curve, Dojo is rich and powerful!