Being able to perform split A/B Testing is something that is vital for ecommerce stores if they wan to improve conversions.
Currently this is something that's really hard to achieve. Being able to select things like displaying a side bar or not, testing different sidebars. Changing the colours of buttons (altering the class names on tags), different layouts (using templates) would be useful for example.
Steven Sashen commented
I'd also like to see the ability to split test functions. For example, let 50% of visitors see, and access, Free Shipping (and whatever messaging is required to promote it)
This would be great