The Discussions tab is actually great. However, there is one thing I think is best to be added there: it should have the option to be placed below the README file. By doing that, it’s just like there is the Comments section whenever you create a post using the README. On a User Interface, we all know that the placements of specifics do really matter. In our case here, others will be encouraged to comment right after reading the content. The current place of the Discussions tab is not that encouraging because you can’t see that instantly, you still need to click that, so again, it’s not that encouraging.
When there is the option to place it below the README, so your content is not simply a discussion of your software project, say, you used it as a blogging site, then others can comment directly below. And that will be more interactive, isn’t it? That’s the very target of the Discussions tab.
Another idea, I think, is even though GitHub Pages is static and others knows this and doing this, they are adding external services to enable commenting, so that the page contains the comment section below. So, my idea is that, if we put Discussions after README and others commented, once a user enabled GitHub Pages, the rendered web page will also contain the comments. If another visitor wants to comment, he/she will be redirected to GitHub, if there is no account, they should create one or GitHub can enable anonymous commenting. Of course anonymous commenting is not that good as it can be abused but as simple as getting the email of someone for identity can be. This way, GitHub is attracting more and more users.