The "Christmas Specials" in particular seemed to be pandering shamelessly to the lowest common denominator. Do we really need to have it hammered home that it's Christmas with references to the season or snow every few minutes? In The runaway bride we had not only Christmas galore but a whole sequence of the Tardis flying along a motorway (did RTD forget what the Doctor's ship actually was?). As if credibility wasn't already strained enough, there was shot after shot including trees clearly visible in full leaf - a sequence obviously filmed in the summer. The 2007 episode included Kylie Minogue (obviously for no reason other than publicity) and the immortal line "It must be after midnight on Earth: Christmas Eve." Did RTD stop to consider whether his dialogue made sense? The following year we had snow, a Victorian setting, Cybermen and a giant version of a Cyberman that was wedged uncomfortably into the story to please the kids, again with no thought of its place in the plot. I know it's important to please the audiences, but does this always have to be done by lowering standards?
Time and again, RTD relies on a "deus ex machina": in The parting of the ways Rose acquires miraculous powers from the Tardis and saves the day. In Last of the Time Lords the Doctor is imbued with superpowers because the people of Earth believe in him, and literally flies out of trouble to "zap" the Master. In Midnight (otherwise an interesting story) the Doctor fails completely and another character has to sacrifice herself to save the day - similarly, in the following episode Turn left, it is Donna and not the Doctor who acts as the hero.
Time and again RTD tells us that something is an absolute and cannot be changed, only to then change it: in The parting of the ways, the Daleks have won and killed Captain Jack (Rose turns back time, kills the Daleks and brings Jack back to life); in Doomsday, Rose makes her exit trapped in a parallel universe and can never see the Doctor again (she returns several times); the Doctor cannot save the Mars colony in The waters of Mars because their deaths are a focal point in history (he saves them).
Possibly the worst of all RTD's stories was David Tennant's departure, where any possibility of emotion was killed by his chaotic plotting and oversentimentality. The whole two-parter had a plot riddled with holes in a story that was meant to be about the return of the Time Lords but gave them nothing to do. Why was June Whitfield's group of senior citizens in the story? Why (other than that it might amuse the kids) does the Master suddenly have the ability to fly? What was the point of reintroducing Catherine Tate as Donna, then to do nothing with the character? How could the Doctor survive a fall of hundreds of feet, then just get up and walk away? Since we've already seen that editing has progressed way beyond the 1970s, why do we get shots of characters pointing a gun at each other and waiting several seconds, unable to decide what to do? If the Doctor is dying of radiation sickness, how can he heal all his wounds and then carry on as if nothing has happened, with time to take a whole series of trips around time and space before regenerating? This was all particularly sad since the performances (with the exception of John Simm) and the direction ranged from good to excellent. I have just one plea to Steven Moffat at the moment - please find a new composer! Murray Gold's music may go with RTD's heavy-handed storylines, but better writing deserves a vastly more subtle accompaniment than MG has ever given us.
I realise I could do no better - I have tried in the past and failed. You might be interested in the letter shown below (a larger version is linked on my own web site). In this, the script editor who was undisputably the "old" Doctor Who's greatest writer outlines to me and a friend the important factors in constructing stories for the programme. Does anyone spring to mind who regularly breaks all these rules?