Force users to tag new threads??
 

How could I do to force users to tag the new thread they create..?

Seems impossible. Even if you were to require text in the field, they'll just fill it in with inaccurate tags, which would be worse than none at all.

Maybe you could give them a tag position closer to their eyes.

As example you can place "Tag input field" bellow the title, when they posting new thread. Above the text (message) area. Example is here (must register).

Sorry about my bad English.

That is what I did. I have a custom form that users use to submit threads in a couple of forums and I really wanted them to start tagging stuff so I put the Tags field right up near the top with some text suggesting they tag their stuff. I've noticed a few more users starting to use the tagging. (Not a huge increase, but more than when I left the tagging below the Attachments option.)