As narratives go, this should be a good one. Arsenal finally lost their media-described FIRST REAL TEST at the third attempt (thanks for making Spurs irrelevant, media!), as Robert Lewandowski, who probably should've been sent off for an elbow on Laurent Koscielny, finished a devastating Borussia Dortmund counter attack.
As matches go, though, this is one where the result, rather than the performance, is annoying. Aside from the first 20 minutes, where Arsenal were firmly second best, they largely dominated the game; had Santi Cazorla's shot gone in, rather than hit the bar/post, the narrative would shift to how Arsenal are coming of age in the Champions League. Small margins. This is by no means a battering in the mould of losing to Schalke or Bayern last season; rather, I think you'll find Arsenal were unlucky.
Yes, there was a tad of naivety in the second goal: Mikel Arteta was deserted by Aaron Ramsey and Tomas Rosicky, with the latter supposed to be deeper, and Bacary Sagna was about 10 or 15 yards behind play. But Arsenal played well tonight--Kosicleny and Arteta were excellent, as was Kieran Gibbs, and despite missing Flamini, Arsenal were not overwhelemed by Dortmund, despite what some may say post-match. It's hard to take too many negatives from the performance. Aaron Ramsey and Mesut Özil weren't at their best, probably because they've played a lot of minutes in recent weeks, and Theo Walcott was missed a lot.
How Arsenal respond will be important; if this is just a blip, like the Aston Villa game, then it shouldn't be a massive issue that Arsenal have to go to Naples and Dortmund.