Many times questions are related to different forums at the same time. For example a question in c# can be related to wpf, Entity Framework, Linq to Sql and so on.
So when I ask a question in wpf only the people in wpf can answer the question, which most probably would have good knowledge of other forums (eg entity framework).
Therefore by putting tags instead of selecting Forum groups the question can reach a wider audience and get a more accurate answer at a shorter time. At the same time posting a question would also be easier. All you have to do to post a question is to fill
up the title, body and insert the tags, rather than scrolling up and down to find the right forum. I am certain that by implementing tags, Microsoft forums would be a greater place to post questions.
Thanks for reading.