Yup, what these guys said. Common problems are way too much dubbing, too many thread wraps and poor thread control, proportions and crowded heads.

Another big one for folks starting out, if they're selecting their own materials without benefit of a mentor/instructor is what to use for different purposes (wet vs dry fly hackle, bucktail vs deer body hair etc), appropriate substitutions (cdc, comparahair, hare's foot) and how to tell good stuff from crap.