You missed Cazorla's influence today. It won't happen every week - we've seen your current system work at Man City and home to Villa, but the fact it was a derby meant that Tottenham played with a unique, fierce intensity you struggle to match. Santi was squeezed out of the game, unable to influence, and you were second to a very determined Spurs side.
Welbeck over Walcott was a shrewd move from Wenger. It worked, too, judging on Ozil's opener. Sometimes there can be a case made for a side scoring too early and I think you were probably guilty of that today.