I still don't understand how a series this short can be that good. There is nothing bad I can say right away (maybe due to the length of the series). Best anime series ever. One of the very few good. perfect? Add 2 more season and then we'll talk.
2014 update: It's not an anime, for starters, sorry about that. I love how the series has explored social and economic ideals so elegantly. However, season 4 sucks. It's boring, repetitive, makes no sense, and seems just like filler. It surely won't end with fans asking for more, I'll tell you that. However, season 1-3 are almost as good as the Last Airbender's series.
Season 1 (Extremism/Terrorism View): 7.5/10
Season 2 (Religious Fanaticism View): 4.9/10
Season 3 (Anarchism View): 8.9/10
Season 4 (Communism/Fascism View): 1.9/10