Are there any specific design principle that is being followed here that stops us from using nested replies like other forums or social platforms instead of being on the top level like what we have now?
The format seems "visually" confusing specially when you are reading from an already long thread and there are several sub-topics being discussed.
I don't know if there's a Discourse extension for nested comments, but sub-topics or sub-conversations is something rather common in forums. We're not linear in nature, it seems 
. I think Discourse does a good job of indicating who's replying to whom, so there's that.
The rest is personal preference; I've been in forums where you could format it structurally or linearly, and I've always found the nested presentation a little confusing and more difficult to follow because it's not chronological, and if you need to reference several "sub-threads", it looks very wrong. It's also harder to follow the updates. So I suppose each format has its pros and cons.
I've personally found nested structures more confusing and unergonomic for conservational forums. For example, Reddit is a pain to navigate and hold conversations in. I think Discourse does very well at making a linear conversation style work well (replies, quotes, splitting topics).
I find nested forums are a muddle. Often nested discussions are a sign things have gone off topic. I prefer to have a single topic at hand and a linear discussion about that. When things go sideways it's time to spawn an entire new thread.
What I would like is paginated threads rathe than the infinite scroll we have here. Is it just me that finds it impossible to search for anything in a thread here?
There’s actually a new, experimental nested replies system on Discourse! AFAIK, it’s kind of unstable though.
I can agree that sometimes nested discussion could mean things are going off topic. But there are cases when tiny specific details related to the post creates a series of discussions that may or may not lead back to the main topic. I think in this case, having the freedom to decide whether it is linear or nested may come in handy, but that will be an unnecessary burden for whoever manages this website.
Second that. Moderators here are pretty good at moving off-topic subthreads to separate topics and that's much better for the conversation.
Dealing with centithread on URLO is pain, but compared to Reddit… it's nirvana. Because with an active discussion you don't want to scan dozens of three to find new messages, they arrive neatly the bottom — and when there are more than few hundred messages and few separate discussions… moderators come to the rescue.
Yes, relying on fellow humans help… how quaint.
The difference between a public forum and a chat room is that most of the people who read forum posts are passers-by, not participants. Tree-style replies allow people in the future to skip over the parts of the conversation they don't care about.
From what I can tell the developers of Discourse are indeed philosophically opposed to prioritizing bystanders at the expense of participants, so the difficulty of navigating large threads is intentional.
That sounds weird to me. Why have such a philosophy? For most threads here I am a bystander, I will not be contributing, That does not mean I'm not interested in the discussion and sometimes learning things from it. Why would they want to make that difficult for me?
If it's harder for people to sustain meaningful discussions, then there will be fewer meaningful discussions, which ends up hurting everyone, including bystanders. In any case, I think the philosophy of prioritizing discussion in a discussion-based forum is logical.
Perhaps it's a reasonable principle to hold, but I don't believe that forum software design has already reached a zero-sum game between participants and bystanders. Developer effort could be more constructively spent, by making various attempts to improve the experience for one group without proportional expense for the other. Discourse is pretty good at what it does, but I wouldn't say it's so perfect that it has no room for change anymore.