Why not use a polyfill to get the same functionality on older browsers? That also gives you the benefits of better performance on browsers that do support the native version.

jQuery uses getElementsByID, getElementsByClassName etc. when available in the browser. There is only a small overhead in parsing the selector to see if it's e.g. a simple ID or class selector.

Or should create a DOM element. I think the main pitfall of $() is that it tries to be too many things at once. Node creation and selection should be two separate slimmed down functions. There's no need (other than convenience) for selection and creation exist in the same function.

And onDocumentReady handling :)

You can do this:

    $(function() { /* handle onDocLoad */ })

Exactly, there's too many if this than do thats in $().