This wasn't bad - it was certainly a lot more dramatic than either of the last two episodes - but it wasn't that good either. What let it down, for me, was not so much that the basic plot structure and resolution was something we have seen many times before - I'd consider that to be an acceptable, if cliched, housing for the meat of the story: Doctor-as-Marshal in the Carry On Cowboy set piece climax. No, what let the episode down for me was that... well, how can I put it?
The character of the Doctor in the 2005-2??? "New" Series is, when the writers remember, that of a psychologically damaged war survivor, who needs humans (preferably women, but Bernard Cribbins will do at a pinch) to act as his moral compass. This is fundamentally different to the character of the Doctor in the "Old" Series, and it means that we now quite often see the Doctor act in ways that would have been unthinkable in the period between Hartnell's Doctor learning human morality from Ian and Barbara (it would have been quite a different show if the Doctor had routinely solved his problems by staving people's heads in with rocks) and Eric Saward getting his hands on the Script Editor's job.
The moral dilemma that is one of the iconic scenes of Genesis of the Daleks sees Sarah and the Doctor debate the rights and wrongs of destroying the Daleks in their infancy. Sarah argues for their destruction, but the Doctor cannot do it. Nowadays, the character of the Doctor and the relationship he has with his Companion necessitates that it would be the other way around.
RTD, who introduced the "psychologically damaged war survivor" character traits to the Doctor in 2005, seemed to close off that arc when the Doctor refused to destroy the Daleks in The Parting of the Ways, and if that had been the case then it would have made sense and been a satisfactory closure.
But he couldn't leave it alone. The very next story saw the new Doctor say he would be a man of "no more second chances," and a year later we see the most definitive statement of intent in The Runaway Piece of Shit - the Doctor "needs someone to stop him."
Every time this character trait resurfaces, and in A Town Called Mercy it was particularly dominant - especially in the scene where the Doctor is about to give the villain up to be killed until Amy stops him (even the compassionate Rory was acting out of character here in supporting the Doctor) - I am reminded that this isn't Doctor Who, it's just A TV Programme Called Doctor Who.