No this is not a subscription based website. You need to have a certain reputation for posting comments. This is simply to ensure the quality of the site.

As @DavidPostill suggested. Please refer to the help section:

[What is reputation? How do I earn (and lose) it?][1]

[Why do I need 50 reputation to comment? What can I do instead?][2]


  [1]: https://superuser.com/help/whats-reputation
  [2]: http://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead/214174#214174

*Posted this as an answer in the light of making sure everyone else interested notices it.*