Visual Website Optimizer started as the easiest A/B testing tool (it still is!), but there's a lot of features now. From cross domain tracking (that requires 3rd party cookies) to geo-behavioral targeting with 15+ variables to heatmaps. It's actually becoming a very comprehensive analytics and optimisation platform. Here's the full feature list http://visualwebsiteoptimizer.com/feature-list.php

Not sure if I answered your question, but the gist of my response is that features add lines of code. There's frontend, backend, test cases, housekeeping scripts, monitoring, etc.