Spend some time in jQuery and you'll see how much faster you can move on the DOM, and with half the code.
I don't mean to say it replaces JS, because you will always have native JS. But I would recommend not doing stuff like DOM and CSS manipulation and AJAX in pure native JS. jQuery extends JS and acts as a compliment, not a substitute.
But, I think we both agree that JS doesn't get anywhere near the credit it deserves.