I'm now watching the entire series and so can say a few things based on the first three seasons..
Series 1 - Budget was too low but Captain Jack is brilliant, they should bring him back even if the Doctor does now know that he's the Face Of Boe
Series 2 - Pretty good, the episode around the black hole was very well written, the Cybermen should feature more heavily in the show
Series 3 - Was not a massive fan of Martha but the series was extremely well written with a marked increase in the quality of the episodes compared to the first two seasons. John Simms is the perfect Master, they should have dragged him back. Blink is (i say this as somebody who does not really do emotion for TV) an absolutely brilliant episode in terms of actually being a bit chilling. They should feature the Weeping Angels more.
Also i hate the new opening sequence with the clocks and stuff, the one for series 3 is brilliant.