A lot of people are massively underrating Koscielny, lumping him together with the bad reputation we earned by having the likes of Senderos, Almunia, Silvestre, Squillaci, etc. Those who have seen his stats, realize that he's improving every year and is close to being world class. The first half of the season when he was injured/on the bench, we had a terrible record with the Vermaelen-Mertesacker partnership and were sat closer to relegation than the top 4.
When Kosc started playing, we suddenly had the least goals conceded in the premier league and climbed up to 4th by scraping 1 nil wins... most of which involved Kosc saving our asses with a last second tackle or interception. When Mertesacker would go forward, Kosc would cover for him, and when he saw the opportunity he would always try to start offensive moves. Another thing people don't know, is that he's one of our fastest players, only edged out by Theo and Ryo.
EDIT: took me a while to find a decent compilation without ear-bleedingly horrible music (skip to 1:10 to avoid the bullshit intro)