From my memory of watching many of the shows as a kid, it goes like this (not being a fan, or watcher of the revival):
William Hartnett: The writers try to establish the still relatively new popular sci-fi genre. The good doctor is initially presented as a grandfatherly figure of great learning, with innocent two-dimensional kids making up the companions - very much a staple of the time. The BBC being at this time is still very much bound in British stiff-upper lip convention - it is generally reflected in the scripts, storylines and 'class' of characters in the show. The storylines are very cutting edge for the time, and still very well written none the less.
Patrick Troughton: We're up to the mid-sixties and the 'liberal' age is beginning to show through with relaxations in the style of writing, and storylines. The BBC is still bound by 40 years of tradition, but is slowly relenting to the society around it. Comedic overtones in Troughton's character provide a welcome change. This, I guess, from what little I've seen of David Tennant, is as close to his character than any of the other Doctors.
Jon Pertwee: Britain hits the Seventies. The peace loving sixties die out, and liberalism is slowly replaced with discontent as Britain slowly works its way up to the punk age. The BBC becomes far more liberal, as if it is about 5 years behind the rest of society. Pertwee plays a much harder, physical, action-orientated no-nonsense Doctor that makes for some of the best stories
Tom Baker: The first Doctor I remember as a child (I have a split in one of my toenails that was formed after dropping a marble onto it when watching a Tom Baker episode - memories!). Tom Baker's unique style is massively imprinted onto the character, taking the comedic elements that Troughton's wonderful acting style introduced, and turning the volume WAY UP. The eccentricities that are now seen as staple Dr Who characteristics are ground into the show around this time. The productions levels plunge, but the writing remains generally excellent. The BBC looks as if it really can't be bothered with making the show any more in some shows.
Peter Davidson: One of the weaker actors to play the Doctor, Davidson's fairly vanilla approach to the job was pretty underwhelming. For the first time, the villains start to become much more interesting than the Doctor. There is a constant feeling amongst the show's younger fans that Tom Baker should never have gone. The production levels, however, seem to increase, as an understanding of 80's technology helps.
Colin Baker: By now the BBC are going through the motions. The show is still clearly popular, but by now it seems just about any jobbing actor is up for the role - probably owing to the fact that the Beeb would rather spend the money elsewhere. I was 7 when Colin Baker took over, but still remember the previous two Doctors much more vividly, because myself and all my friends just didn't like Colin Baker. It didn't help that he didn't bring anything to the party - he just seemed to try to play it the same way as Peter Davison.
Sylvester McCoy - By now the low budget the Beeb always wanted to throw at the show was beginning to show more than ever. The Sci-Fi genre had vastly increased through movies and US shows throughout the late 70's and mid 80's and the Beeb just couldn't compete. McCoy tried to play it both comically, but also tried to bring a harder edge to it when he tried to bark commands and play EMOTIONAL, but it just didn't feel right. The show eventually became a casualty of it's own time. Red Dwarf had come out at this time, and was getting a better reception, and was also demanding a decent budget from the Beeb. It couldn't sustain both show, and canned Who for nearly twenty years. Not many people seemed to miss it either.
There - that's my view growing up through it, and having watched many back shows during the late 80's. I watched the Ecclestone series on and off, and didn't get on with it - I watched slightly more of the Tennant series, but it just doesn't do it for me anymore. There are other people here who can comment on the recent shows more freely than I.