Is tagging is the best user friendly way to categorize a subject?

I say yes!

Tagging provides a many to many relationship between questions and categories. It makes them loosely coupled and so gives control as well as flexibility for categorizing things.

What you want depends on the nature of the media being categorized.

If you're working primarily with media that is difficult to index, like images, audio, or video, then you want a loose tagging system that encourages putting as many tags as possible with each item. The more tags the merrier, because you will need to use the tags to help index the content for searching.

For something that is more easily indexed (text!), you want a much more rigid system where it may take an extra step or two to create new categories or even force users choose from pre-defined categories. You no longer need to rely on user-supplied tags for search indexing, because you can index the content directly. You are strictly doing categorization, and for categorization to have meaning you need to be sure that users are sorting things into the same categories.

Whether or not a hierarchy or tree structure is appropriate depends on how well your categories fit into a tree structure. Some things fit better than others, and many things that appear to fit a tree structure (like programming topics) turn out not to be such a good fit after all.