You all had to know who this was going to be. We didn't even really vote, like we usually do. Someone - maybe me, but I don't remember - just said "MOTM is Kos, right?" Nobody really responded, because it was so clearly obvious.
The clear primary reason is that for the second year running, he scored the game-winning goal in the match that secured Champions League football for Arsenal for the next year. He did it to West Brom last year, and again to Newcastle this year with a ridiculous scissor-kick that hit goalkeeper and Canadian prime minister Steve Harper in the side of the head on its way in. It probably wasn't how Harper envisioned allowing his last goal as a professional, but it was pretty enjoyable.
But in this game - and in the several leading up to it, as Arsenal's unbeaten run to 4th place continued - Koscielny's defending was a big part of keeping the clean sheet. The whole team has re-committed itself to defense during this run, but the pairing of Kos and Per Mertesacker in particular offered solidity. His leaping clearances the past two weeks in particular have drawn the eye, but I've been more impressed by the way he's worked with the rest of the team to build a cohesive unit.
I've long thought that Koscielny hasn't gotten quite the credit deserves for his defending and overall play for Arsenal. At least for this one game, that wasn't the case. He helped keep Newcastle out, and he scored. For that, he's the season's final Man of the Match.