My experience: GitHub (and Bitbucket) are great. Search engines find relevant discussions, or you can browse the repo to see everything. Everyone's comments are on one page, and it's obvious how to add your own (there's a giant green 'comment' button). It's dead easy to make an account--most community people already have one--but if not, it's a familiar process. Thus, GitHub is very welcoming, and levelling too.
Mailing lists, on the other hand, are dire, and I believe excluding to almost all of the community. From a search engine, you'll be taken to a single post from the middle of the discussion. You might see excerpts from earlier posts in reverse chronological order. It's disorientating. To read all the comments, you have to click around a tree structure (shown on a different page). That is ludicrously unfriendly. Most people surely give up. Perhaps the experience is better if you are a member of the mailing list, and receive the messages in your inbox. Understand though, that new people will always read mailing list posts on the web.
Worse, there are no instructions on the page how to comment. For example https://mail.python.org/pipermail/distutils-sig/2013-August/022529.html , there's no big green comment button. If you click around, you can sign up to receive future posts in your inbox (not sure I want that) but that still doesn't explain how to reply to the post you're reading. I suspect older community are oblivious to this problem, because they grew up on mailing lists. Understand, most young people don't know how to use a mailing list--they've never seen one. By modern standards, the process to do so is laughably slow. (I'm reading a post on the web. I should be able to write a comment from the same page). It's prohibitive for many.
PEPs are thus problematic. For example http://legacy.python.org/dev/peps/pep-0453/ . There are no community comments on the page itself. There are links to five different mailing list pages where you can read what other people have said in the past, but there are no instructions how to have your own say. That's a shame. (Compare with Ruby feature requests which are discussed on an ordinary comment thread on the same page https://bugs.ruby-lang.org/issues/8992 )
bugs.python.org is also unwelcoming, in my experience. I reported a bug once, it was closed immediately, with a reply
You can't comment here, you're in the wrong place. This subject been discussed on the mailing list several times before [no link given]. If you want to comment, you should join the mailing list, go back in time, wait for the right post, and reply there.
The message was polite, but completely unhelpful. I felt unwelcome. I gave up trying to contribute to (what I assumed was) the community and started writing rants on Stack Overflow instead. They proved extremely popular, which encouraged me to try participating again. I'm happy I found your GitHub group--you have interesting things to say, and you've accepted my contributions. Thanks. I've even submitted pull requests--I love how levelling GitHub is.
To clarify, I don't believe the poster on bugs.python.org was trying to exclude me--they probably thought they were being helpful. But the perceived "we do things our way" attitude made me feel ignored and unwelcome. I'm sure other potential contributors have turned back at the same or earlier hurdles. http://bugs.python.org/issue16675