About who decides: Money talks. On the first level, it's SP holders maneuvering to assign value. On the second level, the market is some signal of approval for what is happening on the platform.
If the SP holders don't do a good job, we are all doomed :). That's how I look at it. About how the market values? Well for the platform to be valuable to potential social media advertisers, there needs to be people there. People leave if they all think steem is a rat hole with no hope of improvement.