Doctor Who is a show all about time-travel, so the Doctor can experience events in very complicated ways and their timeline might not always give them the quickest way to a resolution.
Look at River Song. The Doctor met her at the end of her life when she knew him very well. They dropped in and out of each other's lives for years and it was, to put it simply, complicated.
There's such a rich history in Doctor Who which allows current writers to look back and make as many connections as they want to the past. For example, in 1983's The Five Doctors, the Doctor steals the seal of the High Council from the Master. It wouldn't be until 2013's Time of the Doctor that he would actually use it, and remind us that "I nicked it off the Master in the Death Zone".
With a new series on the way and David Tennant, Catherine Tate, Bernard Cribbins and the rest of Donna's family all confirmed to return, it looks like we might finally be getting a pay off to the story where Donna couldn't remember the Doctor without potentially dying; a story that was set up 14 years ago!
10. RTD Arcs
In season one, we keep seeing the words Bad Wolf, in season two we had Torchwood, in season three we spotted Vote Saxon posters, and in season four we kept being told that planets (and bees) were disappearing.
Each of these mysteries was developed over their respective seasons, with information slowly being gradually drip-fed as we progressed through the story. The arcs would all last for an entire season and then hit us hard at the end.
The best of these arcs was seen in season four. Multiple threads ran through the episodes; the disappearing planets and bees, the return of Rose Tyler, mentions of the Doctor Donna, the Medusa Cascade and "there's something on your back". All of these things would return towards the end of the season to bite us on the backside when it all came together.
Season four was an absolute masterclass in storytelling from Russell T Davies and his team, and these arcs are something we're really excited to see when RTD returns. We can only wonder what he's got up his sleeve, but we know it's going to be good.