Can you describe what he can do exactly? Is he more than just a guy who's good in the air as well as defending?
I'll tell you a cool story about Sebastian that many don't know about.
When Seba (short for Sebastian) was growing up, he played as a defensive midfielder for Nacional's youth team. His performance wasn't that bright and he was about to be cut lose (¿loose? - Haven't spoken much English the last few years) and the P.E. coach recommended him to try playing as a CB... He was doubtful at first, but he wanted to make it to the A team so he started playing there, and as everybody knows now the results were great.
Seba has great airplay (he should have scored against Paraguay, but his header was stopped by some opponent's hand (<a class="postlink" href="http://www.youtube.com/watch?v=Sm1WyKjl-Es&feature=related" onclick="window.open(this.href);return false;">http://www.youtube.com/watch?v=Sm1WyKjl ... re=related</a> 00.37) in both attack and defense; we all know that.
Things to point out: He has great personality, he is a leader inside the field, and isn't temperative; thing that also helps. He is humble and pretty smart, thing one can notice by watching him play. He also has decent ball handling ability instead of being the regular flipper cb. Seba is only 20 yo and has around 15 international games with Nacional, and has been called to defend the NT for the first time in 2009 before the WC.
That's it for today, have a nice weekend :)