I'm often told that some things are impossible to measures. It's true that some things are much less obvious but there are some pretty ingenious ways around this. If you can't measure something directly then you can look for "coincident" events or activities. What am I gibbering on about? Let me give you an example:
How do you measure how much kids like a film? If you have children, particularly young ones, you know that just asking them isn't a terribly reliable or simple way of measuring whether you have a hit on your hands. The Children's Film Foundation (now defunct I believe) used to count the number of children who used the toilets during the film. They had noticed that the more riveting the film the fewer children used the loos. Of course there are some factors here that may cloud the measure, but it's a great way of measuring something quite subtle in a quick and practical way.
Next time you start to fill with gloom because something is "too hard" to measure, have a think about whether there's a "coincident" event that may help you.
Once you have figured out how to measure something, your next challenge is how to motivate and incentivise your team to move the needle on that metric!