Infants appear to have a mental life that is significantly different from children or adults. They may not have what a child or an adult would call free will, but they lack the means to tell us and they forget what it was like as they become children and adults.
Infants, children and adults who are asleep appear to have a mental life that is significantly different from that when they are awake. People who are asleep may not have what a person who is awake would call free will, but people who are asleep lack the means to tell us and they forget what it was like as they wake up. People who sleep uncontrollably (due to narcolepsy, being drugged or for other reasons) might have even less free will during that time.
People who take drugs inducing psychosis, again, appear to have a mental life that might not be what a straight person would call free will. Some mental illnesses involving compulsion, same thing. And again, people on drugs or with mental illnesses may not be able to convey to others what it feels like.
People with brain damage or especially low IQ - same.
So in each of these cases, experiencing a lack of free will includes the inability to convey what it is like. Thus, it would feel like not being able convey what it is like. Which I'd guess might be frustrating or sad.
Drug-induced mental states may be hard to describe, but people try pretty hard. Below are a few quotes from Erowid.org "experience reports" in which people describe not feeling free will. I don't see much of a common thread between them.
...It is a very zen thing; everything seems so simple and just so. The beautiful, illusory nature of ego consciousness was just so obvious, so plain to see and easy to understand. In the absence of time, the paradox of free will and determinism vanishes. Life is a wonderful game, a grand, extraordinary drama and al
Given the spike in free-will debates on LW recently (blame Scott Aaronson), and the usual potentially answerable meta-question "Why do we think we have free will?", I am intrigued by a sub-question, "what would it feel like to have/not have free will?". The positive version of this question is not very interesting, almost everyone feels they have free will most all the time. The negative version is more interesting and I expect the answers to be more diverse. Here are a few off the top of my head, not necessarily mutually exclusive:
Epistemic:
Psychological:
Physical:
For me personally some of these are close to the feeling of "no free will" than others, but I am not sure if any single one crosses the boundary.
I am sure that there are different takes on the answers and on how to categorize them. I think it would be useful to collect some perspectives and maybe have a poll or several after.