Instead of creating a new XmlHttpRequest object for every request, this post provides code to pool these objects together and reuse them. This saves a great deal of processing times, particularly in IE.
Christian Heilmann does a tree menu without using loops. It demonstrates the flexibility and power of Event delegation - catching events at a higher level in the document. It drastically cuts down on the number of events you need to add to a document. One event handler per menu, rather than one per link in the menu.
Michael Mahemoff runs into a problem with Protypes $$ function when looking for class names. Its taking over 90 seconds when more than 100 matches are returned. The advice is: Avoid $$(".classname") on large DOMs.
Christian Heilmann demonstrates delegating events, and compares it to the main alternative of assigning event handlers to each individual node. Its a technique that's important for writing applications that can scale.