Tagging is only useful if you use it with discipline. Look at Wikipedia or Stackexchange, most articles can be determined by 5-6 categories (science - math - geometry - euclidean geometry - metric). Stackexchange has a max. of 5 tags, often you only see 1-2 on questions, which is often pointless, as those tags will appear often in the question/paper title and abstract too. Wasted time.
So, if you decide to create a tagging system, use at least 4-6 tags depending on how interdisciplinary and specialized your paper/link collection is.
Also consider to not only tag by topic but also kind (review, letter, peer-reviewed, experimental results, theoretical analysis, explanation of new measurement method, meta-discussion...), year, personal rating (very interesting article you learned a lot from and should read again from time to time), rarely/often/top cited, new theory/model, strongly discussed in the research community
A last note. I use myself Copernic Desktop Search as a supplementary tool, I download all papers of possible interest (disk space is cheap ;) ), papers I read, will read or maybe will never take a look at. The point is that Desktop Search software often has more powerful search operators and sorting mechanisms than Google Scholar & Co. If you know how to use them, you can save a lot reading and tagging time or tagging at all. You know, if you are smart in using Google & search operators and have a good vocabulary, you don't have to ask a lot questions on internet boards.
Don't use tagging for creating a pure thematic and linear directory structure, if finding again your papers or bits of information can be done by learning a good Desktop Search software. Use your tags in a personal way and remember, the point is not to structure your bibliography like a folder directory for categorized files, the point is to find again the bits of knowledge and most memorable papers, which will rather look like a strongly interconnected nonlinear tag cloud. If you look how people tag sites on del.ic.ious, often only 2 or 3 tags, sometimes using up to 10 pure thematic redundant tags, they are doing it imho wrong and waste a lot of time.