I think John is mainly worried because Sherlock has become so famous. Of course it was due to his blogging but he is probably aware that this might become dangerous. I think the scene when they leave the house before driving to to court is a sort of foreshadowing of the oncoming darkness. 

I think there have been theories on other threads that right from the start Sherlock was deliberately putting himself out in front of the media by solving all those high profile cases we see at the beginning of TRF in order to lure Moriarty out into making his move.  In other words, it was all part of his and Mycroft's plan to get the ball rolling toward the endgame of destroying the Moriarty network.

John is kept in the dark (as usual) and is tense because he knows what Moriarty is capable of and knows from the pool scene that he has threatened to kill them.  He wants Sherlock to behave at court so that Moriarty will be put away for good.  Sherlock, on the other hand, knows what's coming and what it probably means for both of them. The look he gives John in the mirror while they are getting ready foreshadows Molly's, "You look sad, when you think he can't see you." From that point on you can see him go very inward and self-contained in order to concentration on what's coming.