I just recently signed up to a membership community mostly to try Circle, and I have some thoughts on this. I hope this (admittedly lengthy) post is useful to you and/or the Discourse team because I do think there are things that could be learned from Circle and which could potentially improve Discourse and its broader adoption.
Context (About Me)
In evaluating and understanding someone’s opinion and experience, I think it can be important to know where they are coming from. So for a brief bit of context, I have been a Discourse fan for many years, but only recently (~1 month ago) implemented my first forum in it, using Communiteq (formerly DiscourseHosting) (which made it very easy). I have previously setup and administrated mainly Simple Machines Forums (years ago and since then just maintaining, I have no great love for SMF 
), and I have experience with many others (including vBulletin, phpBB, IPB, Flarum, etc.).
More broadly, I’ve been in IT for 20 years, I started using computers when I was a young child (dad was a computer programmer). And ever since then I have been a general tool enthusiast, often testing new desktop and SaaS apps that I become aware of even if I don’t have immediate need or use for them. I just like new tech and ideas.
However, though I began on DOS many years ago, I have more and more avoided the command line since then, and as it has become (arguably) less necessary to (my) daily needs. Nowadays I’ll admit I can get a bit overwhelmed with highly technical setup/install or configuration processes, or text-only tools. For instance I’m used to markdown by now, but I still wish Discourse had a WYSIWYG editor. And this is also why I used a managed host for Discourse rather than Digital Ocean.
So hopefully that gives you an idea of where I’m coming from. I’m very technically literate, but also very much appreciate ease of use, simplicity, good design, and yes even aesthetics. 
You could say I’m half-way between the Discourse tech crowd and the Facebook crowd, perhaps…
Circle.so
For the most part I love Discourse, and it comes out fairly well in the comparison with Circle in most respects. It is certainly far more configurable, extensible, flexible, and does many things better. But there are a couple of things in Circle worth talking/thinking about and potentially learning from.
First, the cons of Circle, briefly. In general I agree with many of those you pointed out: lack of a drafts system, lack of real quoting features (you have to manually copy/paste), the short snippet+“read more” reading behavior (awful), etc, etc. I would say it’s more concerning still that your comments from 2018 point out these things and they seemingly haven’t improved it since then. It also lacks some of the Discourse niceties like One-boxing.
Now, for the pros, and a quick caveat: some or all of this may be possible to recreate in Discourse with a combination of themes and plugins. I am not an expert in any of that (yet), but I do use a number of Discourse forums on an almost daily basis (in addition to the one I help run), and I have yet to see one that really comes close to addressing the below advantages in Circle…
Design/Aesthetic
The biggest thing that I actually like about Circle is not really a functional aspect at all: it’s the design. I haven’t fully articulated in my mind exactly what I like about it, and there are definitely aspects I don’t like too, but overall I can see how people find it more “friendly” looking and “easier” (seeming) than Discourse. And, as I said, that goes for pretty much every Discourse forum I’ve ever used, regardless of theme.
Yes, it is Facebook-like in some respects. But I quite dislike Facebook Groups, and find Circle.so to be a notable improvement on them somehow. Unfortunately I can’t explain exactly what is an improvement, at least not entirely, but I do want to stress that it really does feel different and, to me, more “inviting”, more “accessible” somehow.
As far as specific design aspects that I like, the main things I can point out as positives are the categories/channels along the left side, and the user list along the right. It makes it look somehow “friendlier” (for lack of a better/alternative word to this already over-used term), and seems easier to navigate - I feel less “confined” when I’m in a topic. And while I like the Discourse approach to threading to some degree, Circle does have nested threading which I do think is simply more intuitive for some people. See example here:
Finally, the “write a comment” text field at the bottom is an interesting difference that makes it seem to invite comment more readily and with a less “heavy” feeling for engagement to me. When you click into that text area it opens a full editor, just like Discourse, so it’s actually no different, i.e. it’s not a chat-like system. And posts within it tend to be every bit as in-depth and fully formatted as in Discourse, if not more so (at least in the 1 community I am evaluating it with). Yet the way it is designed makes it feel easier to engage with, to me.
Now as I said, I know you can do a lot with themes, and I’m sure some of this could be done easily in Discourse. But I think it’s worth the team considering either an official theme and/or configuration option(s) that default to a less sharp and technical look (I wish I had better words to describe it). Because in a functional way Circle is actually quite similar to Discourse (i.e. we’re not talking about trying to make Discourse into something it’s not), and Discourse is ahead of Circle in many respects, while not actually being that much more complicated. But Discourse simply feels less accessible to me (and many others) somehow, and I’m a Discourse admin!
There are many threads across Meta here that talk about user feedback like this (mostly third-hand from Admins trying to compete against FB groups), and mostly in the ones I’ve read the conclusion tends to be “if they want Facebook, let them have Facebook” (i.e. “Discourse is not meant for them”) or “ease of access is king”. However I do think there is another set of users which could easily love Discourse, and with Facebook login the access would not be a huge issue - these people simply get easily tripped up on the design and potential (apparent) complexity.
So what I’m hoping to encourage here is consideration that there may be ways of reaching a broader audience of people interested in high quality asynchronous discussion, which could (and arguably should) happen in Discourse, but is not because of something that may be relatively easily addressed. Simply tuning Discourse to appeal aesthetically to a different type of user may be possible (at least as an option).
Other Circle Pluses
- Default Category: you can assign a category for people to be dropped into when they visit the forums, e.g. Introductions.
- User-creation of “Channels”: regular users can create channels, basically sub-categories, and they get listed on the left side like others. Believe it or not I don’t see this being abused so far, and it’s a cool way to involve the community.
- WYSIWG editor: I know the arguments against this, and I still like it, and certainly it is appealing to less technical audiences, so I do consider it a pro, despite the drawbacks.
- Cleaner, more easily interpreted profile views: this is probably small and again can possibly be adjusted, but I do find the default profile views in Discourse to be quite cluttered with info most viewers probably don’t care much about. I can provide a screenshot of the Circle.so profile page for comparison if desired.
Wrapping Up
Ultimately Circle.so is a fairly limited, closed platform in comparison to Discourse. It offers little functional advantage, and numerous disadvantages. And yet they have a growing user base, and I think it’s worth asking whether Discourse could (and should) have a piece of that market. They are not fundamentally different types of platforms, they are aiming to do pretty similar things in fairly similar ways, but Discourse appeals less to their core market right now.
Discourse is a better system overall, I have no doubt of that. And I chose to use Discourse to start my own community in good part for that reason. But… I also wish Discourse were able to (optionally) be a bit more aesthetically accessible, and without admins having to experiment or adjust a lot of things to make it so. Some kind of “easy mode” or even (shudder) “Facebook emulation” could actually go a long way toward bringing even more people onto the platform, for the benefit of everyone.
I know the Discourse team is very thoughtful about their product and its current approach is very intentional. But I also know they have occasionally changed their minds to some degree on fairly major things over the years. So I hope this is just one more nudge for them to consider the possible need for improving the design further, at least for certain contexts/audiences. And, critically, these are audiences that would absolutely want to use Discourse if the design and UX appealed to them; these are not people who fundamentally don’t want a Discourse-like system or experience. Bring them into the fold!