Why is Prototypejs so great?
For one, it’s no doubt the easiest framework out there. You can use $(’element’) instead of document.getElementById(element). I know that I’m lazy and I prefer using this :).
And going off of an example on protypejs.org, $(’comments’).show() will take an element with the id “comments” and show it if it was previously hidden.
JSON ( JavaScript object notation ) is a fast alternative to XML for transporting data via api’s that is commonly used by many sites such as Vimeo and Twitter . Prototype has a great JSON encoding and parsing support. Used in conjuction with the built it Ajax capabilities that Prototype has, Ajax is relatively easy 
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Oh yeah, that reminds me of how great Prototype handles Ajax. It offers a simple cross-browser way to address ajax. How about: new Ajax.Request(’/some_url’, { method:’get’ }) ? I would much rather type 3-7 lines for an Ajax function that 20 plus that does the same thing.
I’m not even going to discuss Jquery here. That’s for another day, another post.
-Justin B