On the ops side, /r/selfhosted and /r/homelab often come up in searches with pretty good content. /r/aws is also okay, but there's obviously also a lot of certification and amazon event noise. On the dev side, /r/reactjs as you mentioned is pretty good, and I can't really speak to any other languages right now, since I've been full stack js for over a year now, and /r/node and /r/javascript are not so good.

I believe it has to do with the specificity of the forum, passion of the people contributing, activity of the mods to keep it "on topic."

/r/homelab (I've dipped my toes into occasionally) has people who are passionate about their craft. The people who are interested in setting up a homelab are also likewise interested in investing (time and money) in it.

This differs from things like /r/learnprogramming where the people posting are not invested in it (the community or the craft).

Another difference would be the "who contributes new posts" aspect. With /r/homelab new posts come from people of all skill levels. With /r/learnprogramming they come from people who are at the very basic skill level.

The implications of this is that /r/homelab has peers helping peers while /r/learnprogramming needs people who aren't posting questions to post comments and provide additional material.

/r/programminglanguages has good content. Most of the contributors are hobbyists, and some are just starting out, but there are always good discussions to be had.

Seconded. It may not be the content you want, depending on what you're looking for, but there is a lot of great, informative content on that sub.

The reason I have a reddit account in the first place is /r/rust. It was and still is one of the central discussion places of the community, more so today now that IRC has been killed. /r/linux is a great place to hang out as well, at least if you are interested in FLOSS related things. Sometimes you'll even see someone like gregkh chime in.

I remember at one point the moderators turned r/linux into a place where you could only post Linux news. That's when I left the subreddit, and I haven't really gone back since.

as one of those /r/reactjs mods, very happy to hear that we're doing reasonably well! :)

I think /r/musictheory is good

r/rust appears to be a decent community; lots of very good posts.