Javascript libraries

There seems to be dozens of javascript ajax libraries opening up these days. There was an interesting one featured on ajaxian recently called moo.fx that produces very nice effects within the browser. I’m finding it amazing how far the web developers toolkit has come over the last several years. It used to be that it was very difficult to create a full featured website that would work consistetly in everyone’s browser. Now I think we’re finally on the verge of creating desktop replacement apps on the web.
If you’re careful and design the archetecture from the ground up to do so, it should be possible to start replacing desktop apps on the web. I’m very excited to see what the next year brings on this front and am personally playing with creating some apps as a test bed.
One of the issues that I’m concerned about though is with mixing these libraries. If you want to take one thing from one and another feature from another, what is the impact on the end user. Do the various libraries allow themselves to be cut up into smaller pieces?