I think there's an issue of "can I replace this" and "do I care" when it comes to other sites. Every time a change gets made to Facebook or YouTube people get up in arms, vow to leave, etc. but more often than not they don't because there's not another site that caters to that specific need, or there are alternatives but they're not as well supported or don't have the content users are looking for. I think the other thing is people get very invested in sites that they're not paying a thing for and it almost feels like a personal betrayal on some level and they fail to realize that they're basically there to generate revenue.
All that being said, I think Reddit is an interesting case because it's not just "there's changes to the site that I don't like" it's "I don't like the way you're treating employees that we've come to know and love and we think you're a lying sneak." That human element may be what actually makes this more effective than our usual angry fist shaking.