When thinking about China, probably many people associate this with censorship.
On my trip, I had the chance to see the infamous great firewall from the inside. I haven't done any deeper analysis, but I'll share some thinkgs I've observed. A couple of famous sites (for example Twitter, Flickr) are blocked. Contrary to what many people may believe, webpages that are often associated with Warez (Rapidshare, Pirate Bay) were also blocked. The situation with Wikipedia was mixed. Most of the time, I could read the texts on Wikipedia, but access to the image servers was blocked. At the end of our trip, I couldn't access Wikipedia any more.
I encountered no blocks on less famous sites, although I regularly surf sites that could be labelled politically controversial. Though this probably doesn't tell much, except that the chinese authorities are not very interested in blocking european websites.
Interesting may be that the blocking works on an IP level. DNS resolution of blocked sites still works, but you cannot ping the IPs. I haven't extensively tried to circumvent the censorship, as I had no pressing need for it. The only thing I tried was an SSH tunnel, but that usually wasn't possible as the connection never was fast and reliable enough for a stable SSH session.
Most Hotels and Hostels provide Internet access - but most of them by cable. Usually, in other countries today this is done via wireless lan. My theory on that is that a cable-based Internet access makes it easier to log activity associated to a specific person (you always have to show your passport when you check into a Hotel). But still, we had anonymous Internet access (both wireless and cable) at a few places.
Another thing I'd like to mention is what the (non-technical) censorship did with me. I knew that in China people cannot just write a blog, they need some kind of license for it. I was very unsure what this means for me as a forein traveller. I came to the conclusion that I likely won't get any trouble if I just write about my trip without touching any controversial topics. Although I hadn't planned to write anything, this was always in my mind and probably influenced my writings. There was one time where I self-censored myself. In the entry about Hong Kong, I originally had this part, which I removed before publishing:
Most notably it is a place where free speech is possible to a much higher degree than in mainland China. This makes it a very important place for political discussion about China in general. We saw chinese dissident groups that had their information tables and spread leaflets around the Kowloon harbour.
Not much and I luckily have the opportunity to publish it now.
"in China people cannot just write a blog, they need some kind of license for it.". About this, this fact is: when you write a blog which is controlled by the government, you need a licence. Here "controlled by the government"
means that either your domain or host server is bought from a Chinese company. But you should know that most of the biggest blog hosting websites are blocked in China, too.
As of Hong Kong, I think that's because the government promised to not interfere too much, so they have much more freedom than mainland. But the trend is they are losing it.