I consider filling out the Census 2016 form (in its present state) worrying and potentially dangerous.
I've been harping on a fair bit recently about data and privacy, but after the NSA revelations, it's something we should all be very concerned about.
Why is the Census 2016 form worrying? Because it asks for the names and addresses of everyone staying in your house (usually you, your spouse, and your children (the people most important to you)).
You are not asked to provide anonymous information.
Having names and addresses of the population is one thing (we have that from PPS numbers for example).
However, being able to map those names and addresses to all sorts of personal information (such as your religious beliefs, use of a certain language, what time you leave to go to work at) is problematic at best, dangerous potentially.
For example, pretend we have a government at the moment that we all love and that does no wrong. Well, that's great, why wouldn't you hand over all your personal information to them? So, you do, and think nothing of it.
Now, imagine in 20 years time we have a new fundamentalist government (of any persuasion - Christian, Fascist, Islamic, Communist, Atheist, anything) and they have all this historical information on you.
(Remember, governments change - Afghanistan moved from liberal(ish) Communist to Taliban in only 5 years.)
Then imagine this new government wants to purge all people of a certain cohort (all Catholics, or all Irish language speakers, etc).
You can see how this could be worrying and why privacy is important. And why if we need to fill out census forms, they should be anonymised.