I need a software for my site. The software should be: I post an issue and people post their comments. I do not need something like a forum. Just something where I post topic and people comment.
Look at a directory for PHP scripts, like Hotscripts: http://www.hotscripts.com/. Also see if you can get one custom made for you for $5 or some multiple of $5 on fiverr.com by a PHP script writer: http://fiverr.com/categories/programming-tech/php-development
I think any CMS like Worpdress or Joomla should do the job. WordPress is much easier to install and work like you want it to out of the box.
Since you want to "post an issue" I don't think only a "commenting software" will do the job for you. A commenting software or plugin/script will only enable comments on pages that you have already created.
You will have to add the code manually to the pages you want to enable it (like commentics requires). You will have to keep creating new pages(issues as you call it) using a web editor like dreamweaver or handcode it.
If you are going to go through all that trouble its better to install wordpress and you can easily post "issues" rather articles/posts and comments is enabled on them by default.
Worpdress offers a lot of plugins that can allow integration of comments with social media networks and a lot more
You should use Commentics. Its a open source comment script.
Well, posting a topic and people comment on it is pretty much what a Forum does or any news or blog system for that matter.
If you are looking for something more technical I suggest Trac.