When you're building a a product (and in this case, I mean website) it's always a good idea to look up regularly, try to see the forest for the trees and check whether you can honestly substitute the word 'work' for the word 'progress'. This point is fairly uncontentious - hard to argue against it - but what is the right thing to check against?
If your company is mature, there is a pretty solid metric out there called money - and its various derivatives (sales, ebitda, earnings growth, etc). If your company is less mature, you've got to get more creative.
It might seem reasonable to check work against something that you might call a 'value proposition' or even a 'core value proposition'. I think this is fraught because a 'value proposition' is too wishy-washy for this purpose - even if you invent quantitative metrics that seem to describe 'value proposition'.
Rather, you should take a minute to crystallize what exactly the site does technically - the Basic Business Mechanism - and check work against that.
For example, Judy's Book - especially the portion devoted to online shopping - is a shopping site. More specifically, we are an affiliate site, meaning that we send our visitors on to store websites in exchange for a cut of any sales. Even more specifically, we try to get our visitors to take an action (clicking-through) that sets an affiliate cookie on their computer so that a store knows to pay us a commission.
So, shopping sites are, at root, Cookie Delivery Mechanisms.
Progress for us means delivering more cookies to more people more often in a way that keeps them coming back for more (no bad profits).