In thinking about how to implement my own rating system, I always looked at Netflix as one of the examples. They are of course one of the leaders in recommendation systems; and as a consequence, they're a leader in how to get their customers to tell them what they want.
The reason they changed is simple math, via Lauren Goode at The Verge:
After sitting back for a few months, Yellin said, the company observed that over 200 percent more ratings were logged, cementing the decision to change the system.
But despite this 2x increase in data, It's possible that in a few more years, there won't be a rating system at all - ditched in favor of usage data alone. See this comment from The Verge:
“What’s more powerful: you telling me you would give five stars to the documentary about unrest in the Ukraine; that you’d give three stars to the latest Adam Sandler movie; or that you’d watch the Adam Sandler movie 10 times more frequently?” Yellin said. “What you do versus what you say you like are different things.”
Update: Daring Fireball followed up on this too after his readers shared that they liked the 5-star system; he puts it well:
I got a lot of pushback from readers regarding my post yesterday supporting Netflix’s switch from a 5-star rating system to a simple thumbs up/down system. The gist of the complaints is that some people do carefully consider their star ratings, and do value the granularity of being able to say that you like/dislike something a little or a lot. But of course some people take that care. The problem is that most people don’t, and collectively, 5-star rating systems are garbage.