You ask two questions in one, really. But let me begin by saying that there is a reason why we call it GNU+Linux distributions. Ubuntu distributes free software that is made by other people. What software to use and how to use it, is up to Ubuntu, whereas the projects themselves have their own decision making processes.
For things like Unity, that is basically a Canonical product, Canonical has a large role in decision making. But Ubuntu is much more than that. Firefox, for instance, is maintained by Mozilla, so in that case, Mozilla makes most of the decisions. It's like that for lots of applications and programs. It's usually the ones who make the software who makes the decisions. Based on how that works, Ubuntu chooses what software to use. It would be legal to take software, like Firefox and make a new project out of it in order to become "decision maker", but that very rarely happens in reality. When there is a major dispute over the direction of which a project is headed, it can be forked into two different applications. That would also mean splitting the responsibility of decision making and maintaining the software, which is a big job. Also, if the software is very good and the project is very poorly administrated, it can happen, as was the perceived situation around OpenOffice.org, leading to LibreOffice, for instance.
So, since Unity is Canonicals project, they're in charge. But that doesn't mean that other opinions aren't valued and welcomed. They are. Canonical is at the center of it, but it's still a community project. But if the community splits into more than one opinion, and you have to have only one solution, then Canonicals opinion will be the final word. Usually, though, there is a best solution that people can agree on.
For general decisions about the direction Ubuntu is headed and what to focus on, there is a developer summit right after the release of one version of Ubuntu and another begins. The next starts on October 31th and is where the big picture is ironed out. You can read more about that here: http://uds.ubuntu.com/