Drupal commenting system vs Disqus I like the multiple login possibility of Disqus. You can login with facebook or twitter or google etc.
 On the other hand, comments will exist on a 3rd party server and the package that includes the single login feature costs a lot (that mean that drupal site's users can comment without logging again at disqus system or another).


How I can simulate similar system on my site?
How can users sign in with facebook, twitter, google, openid at my site so that they comment?
Is there any other feature of disqus system apart from multiple login that can be accomplished with drupal modules?


[3. Does disqus makes site slower to load?
4. How can I move comments from Drupal to Disqus?
5. How can I move comments from Disqus to Drupal?]

Edit: I would like an answer possibly from someone that has used both systems as advantages and disadvantages appear in the long run, and it is not easy to decide just at a fast research. I have seen sites switching from one commenting system to another because they understood the disadvantages of their choice after some months.
Regarding question 1, there are several options to do that, for instance OpenID Selector.

Question 2: Yes. I don't know all of Discus' features so I cannot answer this for each and every one of them. However I do know that there are Drupal modules for comment notification, for instance Comment Notify.

Questions 3-5: those are quite specific, I think you'd better ask them in the issue queue of the Drupal Disqus module.