IMO the biggest obstacle is getting enough traffic to run accurate tests in a certain amount of time. Once you are able to run quick tests on EVERY aspect of everything, you can begin to finely tune your traffic and make sure you are getting the most out of it.
If you are having to wait a month or months to gather enough data to get a rough idea on what is working somewhere it could change by then and be useless.
If you don't test or optimize anything you're a moron.
