Mozilla is all standards based, which is great, but it doesn't render those standards as well as IE does. Maybe with some time and effort, the rendering engine in Mozilla will be improved, but right now it has LOTS of problems.
It's not so much javascript itself that Netscape can't handle, as how javascript interacts with the DOM. IE has a much more complete implementation of the DOM than NN4.x. Netscape 6 is better, but still needs work to catch up with IE.
NN4.x handling of CSS and DHTML is terrible. Slow, buggy, limited.
I could go on and on. Go check out web developer message boards to see what professionals are saying about the various browsers...