What is censored and why?

 It would take more than a blog post to explain censorship in China but it is, in a word, pervasive. Today, June 4th, was the 23rd anniversary of the Tiananmen crackdown, in which soldiers fired on pro-democracy protesters and ended a weeks-long series of demonstrations. Censorship was at full-force: the China Digital Times published a list of words that were banned on Sina Weibo (China's version of Twitter) leading up to and on June 4th, and various other terms were censored or blocked in other corners of the internet. 

Earlier today I read a paper on censorship in China that really changed how I thought about it - the main points of it are:

1. The Chinese government censors content in order to "reduce the probability of collective action by clipping social ties whenever any localized social movements are in evidence or expected" (and not, as widely thought, to suppress any content critical of the government).
2. An analysis of censorship patterns can reveal the "interests, intentions, and goals" of the government" AND:
3. Further research and analysis along these lines might result in the ability to predict government actions.

Of course, this isn't all the paper said, but these points were significant. All of this has caused me to view censorship and deletions from a different perspective, especially when it comes quickly, as with the rapid deletion of a post that was simply a totally black image, by the world's most popular blogger, Han Han. What for instance, caused the deletion of the highly popular Sina Weibo account @作业本？According to @idzhang3, this picture of the Tiananmen candle-light memorial in Victoria Park was the post that the user made 30 seconds before it occurred: 

This is an image of the vigil taken from farther away:

What interests me most: 

• How likely does the Chinese government think such content is to lead to "collective action"? Is censorship like this mainly undertaken to prevent such action in the short, medium or long term?
• How could recent research on censorship, such as the article linked above, impact the censorship program, if at all?
• How sustainable is this program of censorship? How successful has it been at achieving its goals in the past few decades since the invention of the internet? And somewhat more pointlessly,
• What would the world have been like in the absence of this censorship?