I find the idea of A/B testing and multivariate testing fascinating. On the surface it may seem like you are just rearranging and adjusting elements on a page to systematically find the best combination. That alone is quite gratifying from a strategic level. I would be more than happy to pursue A/B testing from this angle alone but you also have to consider the human element. Why are certain changes more likely to solicit an action from a complete stranger than others? This data can be used to infer human behavioral patterns and that is what I find most interesting.
This page is a simple collection of A/B testing articles from various sources.