The DOM implementation in khtml was particularly well done. I remember one of the major things Apple did was rewrite kjs (into JavaScriptCore), but from what I recall, it wasn't like they did a complete overhaul of khtml right away.
One thing to consider is that Apple, unlike the khtml project, had a lot more power to steer web standards and that tended to clean up browser impls.
It's a shame that Apple decided to have a pretty arcane development process for Webkit, it was not what people expected when they announced Webkit.
This is a fair point. I believe they milked the, "it's Open Source (Come on In!)" and then would periodically release diff blobs and wonder what else the common folk would possibly find wanting.
This is why Apple is met with groans instead of praise when their open source contributions come up.