I think Doctor Who is usually good per se, but, recently, David Tennant has really been hamming his acting up, and I have consistently believed his side-kicks - Rose, Martha and Donna - were all pointless, poorly acted, and only contributed to any story for the sake of it.
Plus, the enemies are seemingly getting less evil and frightening as time goes by. I mean, the first episode of the last series featured those Adipose things, that just looked like bundles of cholesterol scraped out from my own bloodstream. Bill and Ben were scarier than these little nothings. (Actually, Bill and Ben are truly terrifying).
I'm holding out for the new Doctor, but, more importantly, the new writer.