Although very verbose, The REST dialogues discusses why the REST paradigm provides important benefits when writing web applications, and how SOAP and/or RPC web applications can be rewritten to utilize REST principles. It's only up to Part 3 (of 9) -- I'm going to keep an eye out for the next one!

On much the same note (but much shorter), S stands for Simple pretty concisely sums up why I don't want to touch SOAP with a 20 foot pole.

Lastly, Jeremy Keith provides a short and compelling case for writing web applications as pure HTML while designing to add AJAX as a way to "soup-up" applications as a last step.