What does Q do? Apart from providing a "library for modern web apps" (whatever that means).
Reusable tools - Q comes with a lot of reusable tools you can just drop onto a page, many with their own back end and they "just work" when you Q.activate the page
Not refreshing the whole page - Q has virtual pages loaded with AJAX and slots (content areas) to fill on the page. Q.handle(url) simply loads the url for example and activates all the tools for you.
Event system - you nee to be able to remove event handlers when the page is unloaded or a tool is removed
Integration with PhoneGap, jQuery, touch vs non touch environments, etc.
But the coolest stuff is the middleware. You can literally write apps that get objects like this:
Users.byId(uid, function(err, user) { ... } );
Oh and Q supports socket.io so if you have node running you can update caches via a push so they are almost NEVER out of date!
Furthermore a lot of the time your JS code has a functional programming character, such as "wait until these objects are fetched, then do this". So just use this:
var p = new Q.Pipe(["user", "stream"], callback);
Users.byId(uid, p.fill("user"));
Streams.get(publisherId, streamName, p.fill("stream"))
So basically:
Virtual pages and tools you drop on them, set and forget
Model done via caching and realtime updates, query and no worries. Use Q.pipe to wait for some objects then do what you wanted to do
Connect everything together with event handlers
Boom your app coming together quickly. Other libraries (like jQuery) are for manipulating DOM and other stuff. This library helps you build rich web APPS.
But Q.getter for example is just that easy. Try this:
1) Include Q.js
2) Take any function that gets stuff from any web server and calls one or more callbacks, let's say function X
3) Do this: X = Q.getter(X)
Now use your function :))))
See how it improves efficiency? It caches the result, it prevents multiple requests to the same thing, etc. Next you can tell it what to use for the cache, by another parameter to Q.getter. And finally you can hook up socket.io to update the cache on a push!