It came up in another thread that people thought I was joking when I said that Barbarians are uncomfortable when hedged in by walls, and so your Barbarian character feels uncomfortable when hedged in by walls.

It's right there in the PHB. I was taken aback that some people didn't think it was possible for me to be serious about this. I have even seen a few people say something along the lines of "your character and their beliefs and actions are completely up to you" which is false.

What your character believes in and what they do are not completely up to you. There are a multitude of things you can try to do which your group with respond with "no" then you can either retract it, or leave.

One category of behaviours are ones which are socially unacceptable. The most common one to come up in play is probably attacking of other characters. It could be anything though including racism, etc. You're just not allowed to do that because the group doesn't accept it.

Then we have things which are deemed 'roleplaying rules'. These include the Barbarian example above but also include plenty of things which are unspoken. For example, most groups would probably find it unacceptable if you decided that your character suddenly believes they are from 18th century earth in a standard D&D game. Some might, but generally that sort of deviation from the setting is enough to derail a game so would be against the rules. An offshoot of this might be a character who knows the inner workings of all of the dungeons and such because the player has decided to read the adventure.

The argument I have seen against these 'roleplaying rules' is that it constricts creativity. I disagree. I think creating a unique character/story within the rules is the creative part.

I liken this to improv games. If an improv actor broke the rules/constraints of the game to do something unique it wouldn't be seen as creative, quite the opposite, it would be seen as lazy or unsporting.

D&D is a game of fantasy tropes. I think it is fun to create something unique using those tropes. Breaking them is lazy and the game suffers as a result.

Plus, that special character you made who goes against their archetype isn't as unique or interesting as you think they are. We've seen it all before. The interesting and creative moments happen during play with the collaboration of the group, just like in improv.

Of course, play with whatever 'roleplaying rules' you wish. Houserule the ones in the PHB if you like. Do keep some though, as they are important and enrich the game.