3. Characterisation
The Doctor as we know him is something that just wouldnt be accepted on mainstream American TV. Hed be seen by the idiotic network execs as too quirky and not relatable enough despite the fact that hes supposed to be strange and alien. But because of the prevailing opinion that viewers are idiots and only want to see things that are within their own frame of reference, the Doctor would be significantly more grounded and closer to the average man on the street. Also, assuming that tone would be as I suggested, he would be a lot more action orientated and probably be armed with some kind of laser pistol. Because when you need to gun down wave after wave after snarling monsters, a Sonic Screwdriver just wont do. Also, to create more tension and give it a grittier feel, the Doctor would probably be written as a recovering alcoholic/drug addict like Sherlock Holmes in CBSs
Elementary.