Is MODx capable to...

Basically, I have a goal to create a web-site like kotaku.com and joystiq.com. Both of them are using Movable Type which has some essentials features these types of blogs need: an ability to share the rights with PARTICULAR users to post news, an ability for all users to register before posting comments along with adding avatar and filling custom fields, in other words – user profiles (I think it’s called public registration, and I want only users with accounts to be able to post comments). These are 2 very needed feautures. Can MODx handle that? Maybe some plugins will work?
I’m soo tired searching for a CMS which can do those 2 things. Any advice on any CMS will be appreciated.

MODx pretty much handles this out of the box. The user management is part of the core. To login you need to use either the WebLogin or WebLoginPE snippets (the later one has extended features and may be more what you need).

Install MODx with the demo site and you will see a working example of a blog that works like this.

Look no furthur, you may have found what you are looking for