-For starters, S2 didn't really click with me to begin with, but it seems especially dismal after how awesome this season was. There were a few redeeming moments, but I think it would be a struggle for me to ever watch it in it's entirety again.
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HUGE LOL @ Rawls being a closeted gay.
-There were so many fantastic scenes in this season that I can't even remember them all, but I'd have to say
the paper bag speech is on the top of my list. The whole Hamsterdam thing was brilliant, and it's unfortunate that it seems as though the show is already done with Colvin. I'd have liked to have him around a while longer.
-Cutty was pretty cool, glad that he will most likely still be in the mix. When he was first introduced I figured they would kill him off by the end of the season, but I was pleasantly surprised to see him get out of the game. I was still half-expecting him to pull out on Fruit or Marlo at the end of the finale and go down as a martyr for the youth, but that stare that he gave Fruit was good enough

-I saw the power struggle between Avon and Stringer coming way back in S2 (who couldn't?), but
man was it fucking perfect. The build-up, the execution, the climax, everything. I'm pretty upset that I now have to watch 2 more seasons of the show without Stringer, but that whole arc was a big part of what made the season for me.
-I was happy to see the focus of the case unit back on the Barksdale organization, but now I feel like I did at the end of S1, and a quick glance at the main storyline of S4 does nothing to allay my worries. The school system?

It was great how neatly everything tied together this season, and I'll reserve my judgment until I get a feel for the next season, but I think this show could have worked so much better if from start to finish of the series they followed this "from the ghetto to the mayor's office" theme they had going on in this season. Yeah, the show has always sorta been in that vein, but it felt like all sociopolitical conflict they're constantly pushing really flourished here.
-This mostly just expounds on my doubts about the next 2 seasons, but it seems like they could have wrapped up the whole show this season if they fleshed some minor things out a bit more. I was always hoping that the series would conclude with the downfall of the Barksdale organization, and again, I can't see them coming up with anything that will compare to the intrigue of this season. At the end of the 2nd season you could already see where the plot was headed -- up -- but this time around it seems like it's all downhill from here. Hopefully I'm wrong.
edit:
-Oh yeah, one more thing: fuck Herc right up his New York ass. Him and Carver have both always been pretty weak, but I think that this season there was a dichotomy between the two that wasn't there before. Carver is growing up (a little) and Herc is somehow becoming even more vapid. I hope to God that fucker gets killed off before the series is over.