Displaying a text field for the user to embed media, and then enabling rating (fivestar) for that content; listing all of them as separate writings On a given site's page, which is supplied with the "casting" URL alias, I would like to show some embedded videos with a description field, which users have uploaded (embedded) via a simple text field that Embedded Media Field processed. These would appear as separate writings, signing the user who uploaded this stuff.
There would also be a rating/voting opportunity (fivestar, thumbs up/down, etc.) with the help of Rate or Fivestar module (which one do you suggest?!) to let users vote for the embedded video.

When someone logs in, I would like to show the form which helps embedding (or a link to this form) at the top of the page, and below that, there would be the embedded media from the different users.
So the form would be as simple as that: containing a text field, in which e.g. Youtube URLs could be copied (it would be processed by Embedded Media Field module) and a description textarea to write about the embedded stuff.
The result output would contain these: the embedded media, the description, who this post created and when, and a rating/voting opportunity (five stars, etc.).

How could I do that? What do you suggest for creating a form like that?
Maybe it's not too difficult, but I didn't find the perfect solution, and I couldn't restrict content types to these fields (and with the help of Webform I couldn't add a field of Embedded Media Field).
Why don't you just let your users create a node with this field and a fivestar widget and then you use node_view() to display these nodes in your page.  

Updated:
To embed the node form in a page, have a look at Embedding CCK content type create form in your page, then create a block and put this code in it (select the php format - you have to activate the php filter module first):

if(!function_exists("node_object_prepare")) {
  include_once(drupal_get_path('module', 'node') . '/node.pages.inc');
}
print node_add('store_review');  //put the content type machine name here.


Then on another page you create a view to retrieve the listing of your videos.  

You can also have a look at the Blockify module it may be useful to embed the node form (I'm not sure it'll do the job but can be easy if you're not comfortable with embedding PHP code in a block).