This article details exactly what is wrong with series 5 from a character perspective, and I agree with about 98% of it.
Be sure to read all three pages; the third contains an in-depth description of exactly how a new approach and some rewrites could have made it a far more enjoyable series than it actually turned out.
http://www.overthinkingit.com/2010/07/12/fixing-doctor-who-season-five-edition/