The major dynamic of Doctor Who has always been one of change, after all it IS a show about a 900+ year-old face-swapping, regenerating Timelord with no-fixed-abode, each week an entirely new location, with a new cast of characters. It's because of this dynamic that the series has had such (multimedia) longevity. With series 4, of the new series we see a continuation of this, with the return of Donna Noble from the Christmas Invasion Special of 2006. But, this isn't the same Donna, she herself has changed as a character for having adventured with the Doctor, although she still, thankfully, is not as in awe of the Doctor as Rose in series 1, or in love as Rose in series 2, or struck love-sick with the unobtainable man as Martha in series 3. Donna has a unique perspective on the Doctor and his universe, almost seeing herself as an equal, if unsure of her abilities, but not so completely trusting of the Doctor. As Rose grew into the role of companion, so does Donna, but her journey isn't as simple, she may be the everyman companion (like Sally Sparrow or the men of LINDA). The show's overall perspective has changed, Rose and Martha held the point of view, unraveling the mysteries (for the viewer) of the Doctor and his world, especially with the angry and enigmatic Chris E's 9th Doctor. But now with David Tennant's 10th Doctor the audience knows more about the "last of the Timelords" than any companion, we don't need the companion as an anchor, thus we has Donna on an even playing field, and an excuse to bring back Martha (also changed for her travels)...an maybe Rose?
This season the series has no major catchwords like "BAD WOLF" or "TORCHWOOD" and who can forget "MR. SAXON." Season 4 is bound by other elements, a person, phrase, a prophetic statement/question, a place, building up, so pay attention. These elements aren't clues from this season alone, but references planted going as far back as the first season, or a recently as quotes from the Master in last season's finale, remember "The Crucible," "Medusa Cascade" or the "Shadow Proclaimation?"
What else this season has is loads of great stories from award-winning writers and loads of great performances from award-winning celebrity cameos: Nigel Terry (Excalibur), kylie Minogue, Felicity Kendal, Alex Kingston (ER), Colin Salmon (MI6), Christopher Ryan (the Young Ones), Geoffrey Palmer, Georgia Moffett (the Last Detective) and more. Of course, there are loads of great performances from the series regulars. David Tennant ads so much depth to what was a relatively static character, in the series original successful run. His Doctor still enabling the everyman to act, to be a hero. Along with Catherine Tate, their sense of pathos and comedic timing are nothing short of A-list quality.