Average users really have no say while they're here. There's no real ability to control or even influence how this site is operated.
Leaving, or choosing not to actively participate any longer, probably does have the biggest impact of all of the options available to a user. Discussion forums need participants, and participants leaving over time can have a huge impact.
Slashdot is a good example of this. Various site owners/operators started changing things in ways that users did not like. Many users spoke out, but they were ignored. In many cases, they were subjected to downmodding, especially those who spoke out against bad UI changes or the recent beta site. Seeing that discussing the problems had no impact on the situation, they left. Now Slashdot is a wasteland compared to what it once was, a decade or so ago. The decline in participation may be gradual, but it's very real, and its impact is significant.