What about having users puplishing their own stuff without editing or deleting other peoples blogs?
Can Role Manager help with this?
You don't need Role Manager for that. Designating a user as Author allows her to publish her material, but not to edit or delete other users' material (with the exception of media files, of course). Role Manager would help you if you needed some non-standard roles or wanted to change the permissions for a role already defined.
Hmmm if a person use media in a blog it dosent matter if they delete it in the media gallary does it? it will still be part of the authors post/Blog right? but "just" removed from the library correct?
No. When media files are removed from the gallery, they are deleted from disk. So any posts displaying media that was deleted will show dead links instead.
I am new to wordpress but already hitting some limitations like this I really didnt expect from such a great peace of software with 10000 of plugins
If you read WordPress materials carefully, you will notice that even its developers characterize it as a "personal publishing system" or "personal publishing platform". Note the word "personal"; collaboration was deliberately de-emphasized in WordPress to make its deployment and operation easier for a less technologically adept user. There are products out there that are built on a different set of premises (Drupal comes to mind), but they are substantially more complex.