How Sherlock and John were able to play off each other so well right off the bat. If this show had starred two different actors instead of Cumberbatch and Freeman it would taken longer to develop chemistry between the two leads (With “Elementary”, I still feel that Miller and Liu have yet to actually shine together as a team). As a matter fact I don’t think this series would have worked had Ben and Martin not been casted. Although, I think this first episode was able to establish all the characters perfectly in general.
 
This exchange cracks me up everytime I see it
 
Sherlock Holmes: You're a doctor. In fact, you're an army doctor.
Dr John Watson: Yes.
Sherlock Holmes: Any good?
Dr John Watson: Very good.
Sherlock Holmes: Seen a lot of injuries, then?... Violent deaths?
Dr John Watson: Yes.
Sherlock Holmes: Bit of trouble, too, I bet.
Dr John Watson: Of course, yes. Enough for a lifetime... far too much.
Sherlock Holmes: Want to see some more?
Dr John Watson: Oh God, yes.