It appears to be a common misconception that YUI is going to be replaced by Dojo. Alfresco has developed a new UI framework (with the project name "Aikau") which is based around the concept of AMD (asynchronous module definition). It is true that the Dojo AMD loader is being used and that we are creating widgets that are based on the Dojo widget templates, however it is completely untrue to say that we will use Dojo over and above any other JavaScript toolkit. The purpose of Aikau is not to tie ourselves to any specific toolkit and we are using YUI2, JQuery, Dojo, TinyMCE and other 3rd party JavaScript libraries in our new widgets. We intend to use the best tool for the job in all cases.
I would recommend that you use whatever JavaScript framework that you're comfortable with and that solves your problems, but above everything else you should have a good understanding of pure JavaScript.
Regards,
Dave