>What's the difference between closed and not closed question on SO (except taged as closed of course). 

It's not tagged as such, but rather marked as such.

>When you look at it, one can still down/up vote, add answers, add comments and so on. 

You can't post answers.

>So what's the difference. 

You can't post answers.

>In my opinion if question is closed there should be no activities allowed on this question (adding comments, answers, down/up voting etc) except for reopening. 

That's what [locking][1] [questions][2] is for.

>Otherwise it works like some sort of dummy to me.

[![][3]][3]  
<sub>(source: [socialmediamarketing.com](http://www.socialmediamarketing.com/blog/wp-content/uploads/2009/08/CrashTestDummy-2-8544b.jpg))</sub>  


  [1]: https://meta.stackexchange.com/questions/34815/give-diamond-moderators-the-ability-to-partially-lock-a-post
  [2]: https://meta.stackexchange.com/questions/13180/is-lock-broken
 [3]: https://i.sstatic.net/Nuvht.jpg