On his watch the show has been notable for three things: emotional delicacy, humour and spectacle.
He has brought great intelligence to everything about the show- except the SF. If you like your SF to be about ideas- as I do- this is a big problem.
Russell favours storylines involving the imminent destruction of the whole universe (have you paused to think how BIG the whole universe actually is, Russell?)- a fate that is then averted by the Doctor or his belle de jour getting to push the right button or pull the right lever. I've never been able to square myself to the idea that this is proper drama.
The individual shows we will remember have all been written by someone else- usually Stephen Moffat. Davies has a wide range but it doesn't encompass weirdness, mystery, the uncanny. Moffat, on the other hand, is a master of the twilight zone.
The three best stories of the Davies era, in no particular order of merit have been; The Empty Child/The Doctor Dances (Moffat), The Girl in the Fireplace (Moffat) and Human Nature/Family of Blood (Cornell)
Russell's swansong was the usual dog's breakfast: a couple of quite beautiful, highly charged scenes between David Tennant and Bernard Cribbins, John Simm largely wasted in a role that called for nothing but cackling loooniness, a great chase sequence involving nuclear misiles and a rust-bucket space ship- with Cribbins (oh joy) manning a gun turret- and the rubbishest villain yet in the form of Timothy Dalton thesping away whilst dressed as Ming the Merciless.
The coda- with the dying doctor dropping in on old lovers and companions was lovely.