There appear to be three main bones of contention age, race and gender. Oh boy let's address all three.
First on the point of age why is this such an issue. Some have argued that this will cause the Doctor issues with authority figures (my Lib Dem Friends had better now whisper this too loudly near Jo Swinson, Julia Goldsworthy or Jenny Willott that) well if we could find a 900 year old actor they may not also have been totally satisfied either. The fact is in the previous 10 regenerations of the Doctor there have been juvenile and serious portrayals. In my opinion two of the must juvenile takes on it were carried out my Sylvester McCoy and Patrick Troughton, Peter Davidson though youthful in looks was quite a serious vision of the time lord. However, it can be a useful narrative devise look at the way Douglas Adams uses Ford Prefect's distance from his home world to instill a sense of unease in others. So I'm sure can Moffat et al over come the youthful appearance of the 900 year old persona, indeed there will be some quite comedic and possibly dramatic possibilities to come out of this casting.
I'd therefore argue that the age of the actor is an irrelevance if that actor can act. Yeah sure for the first time since I sat down to watch Jon Pertwee this will be the first time that the actor playing the Doctor is younger than me. So what? I'll be in my forties before he appears on the screen.
We move on to race. Like many when I heard rumours of Paterson Joseph or Kulvinder Ghir taking the role I was kind of excited about the possibility, especially of the former. Of course when Freema Agyeman became the first companion of an ethnic minority Doctor who was taking a well overdue step. The rules of sci-fi and indeed the regeneration of a Time Lord would not (as hinted before by the production team) inhibit a colourful outcome of a regeneration scene.
Sure it would have been good for racial equality reasons but rather than making the casting decision solely on those grounds it has to be made on acting ability. Which while these two fine gentleman are not lacking in as said last night there was a stand out performance that the team kept coming back to.
The final option and complaint related to gender. Sure in the worlds of Sci-fi there should be no inhibition of the Doctor changing gender. Although from history, if memory serves, male time lords have always regenerated as males and the females as females of those we have seen in more than one regeneration. Of course all that may well be able to throw itself out the window if we are down to the last time lord, something that was left a little ambiguous in the final scene of The Last of the Time Lords, all things may well be possible, including changing to the correct gender for reproduction purposes to maintain the race. So could some like say Jennifer Saunders or Billie Piper have taken over the role? Possibly, and it is something that cannot be ruled out for the future.
Seeing as both Billie Piper and Paterson Joseph have already appeared in other persona's in the series I'm surprised that nobody has pointed this out as a reason for not selection them as the next Doctor. Sure Freema came back as a companion and different character from her first appearance, but that was explained away in the storyline by genetics. Would it be right for the Doctor to regenerate assuming the appearance of someone from his past. He does after all appear to have no idea what appearance he will regenerate into. Look at the early look into the mirror, feeling or face or teeth of each new version of our favourite time lord. So maybe there is some part of the genetics that when this process goes ahead inhibits imitation of another being already encountered for although it will not result in the actuality of meeting oneself when traveling through the dimensions of time and space you may well re-encounter such a person and it may well cause fatal shock.
In the end the thing that has to be first and foremost in the production teams mind is can our chosen one act, can he portray the Doctor and will he hold the audience and give our writers and directors something to work with. As it emerged on Doctor Who Confidential last night Matt Smith was one of if not the first auditionee for the part, comment like 'in the first hour of the audition process'. Being first is possibly one of the hardest positions to fill as as more and more get seen you may easily be forgotten if great performances follow. Yet Matt had stuck in the production team's mind through all the others.
So true Doctor Who fans must ask themselves how can that me. Steven Moffat and the others are like Russell T. Davies before are well versed in the lore of the Doctor. These people seemed to have been so taken by Matt Smith's audition that they kept thinking back to it no matter who else they viewed. Surely that is enough for any of us to know that after he spends 6 months working out his particular take on the role that there will be something really special to look forward to once the face of David Tennent morphs into that of Matt Smith.
*Pity I can't link to all the others' Facebook up dates