(a) you have no ability to edit entries (or do anything else that email is bad at),
(b) the mail interface is frankly hard to read, with everything in uniform monospace font and no formatting,
(c) I can't participate or reply on the web interface, I have to sign up for something completely different to participate.,
and (d) because of the way the site is laid out, I can't see how to get the code, etc. from the bug tracker. I would presumably go back to Google and search for org mode and then try to figure it out from there. But that's just a lot more steps to do what could be made easy.
Now, I'm not necessarily super crazy about Github and the like. But for these issues, in my opinion they do a pretty reasonable job.
I don't personally think that Github makes PR formatting any harder; I would expect any contributors to use a git command line to look at what they put in the commit message. But at any rate, that's some user education you'll have to do one way or the other. With the current approach, you've got a bunch of other barriers to entry that prevent you from seeing certain users in the first place.