Dojo (and also Ext, IIRC) have a couple of semi-unique features, being proper data abstractions in the client layer and extendable, hierarchical widgets systems.
I know Dojo best, so I'll use that as an example;
Data abstraction means a separate API layer between widgets (or other classes) and the actual data. All dojo.data datastores (as they are called) have the same kind of API, but manages access to data differently. For example theres one store called ItemFileReadStore, which just read in a JSON object from an url and provide it as a bag of items to widgets.
Let's say that you use this data store to provide data to a combobox which implements type-ahead (which the Dojo one does). After testing you want to have a more advance data store which only fetches some items at a time (because the amount of data on the server is too large to load at once) and caches it. That means switching to the JsonRestStore, which uses the same API as the first one, et.c.
Here are some good articles that give more meat on dojo.data;
http://www.sitepen.com/blog/2008/06/13/restful-json-dojo-data/
The widget system in Dojo let you define custom widgets and place them on the page, using a single element with special properties. Dojo scans for these elements and look up the widget definition (in a JavaScript file and the HTML template for the widget (pointed out by the JavaScript file).
Dojo then calls special functions in the widget class according to lifecycle events, and also manage unique id creation for html template elements, instantiation of subwidgets, et.c.
Dojo uses this system for all its own widgets internally, and many custom widget projects I do for customers start out with subclassing an existing widget (using Dojo's pseudo- OO). The reason this work so well is that Dojo have proper namespacing, which also correspond (but can be overridden) to directory hierarchies, so class/widget lookup is both straightforward and understandable.
At the moment jQuery have no data abstraction or widget subsystem (unless I've misunderstood jQuery UI completely, in which case I would very much like to be corrected :), and neither have YUI.
Now I've done it again. :) This is my next blog post, apparently.
Cheers,
PS