So, I kind of hated AC2. I mean, gameplay wise, it was great, but the story was almost nonexistent, both for Ezio and Desmond, and the ending was more than a little facepalm inducing. I'm still invested in the series, though, and although I'm not ready to dive back into a whole 'nother game just yet, I'm wondering if this is something I'll need to get my hands on at some point before AC3 comes out, just so I don't miss anything important.  
 
So: Is the story just universe expansion, or does it actually move the plot forward? Is it worth playing through for the story, or can I just read the eventual summary on the internet and get everything I need?

I just beat the game...it is critical to the story of the franchise

" Is it worth playing through for the story, or can I just read the eventual summary on the internet and get everything I need? "

If you hated ACII, I don't see why you'd want anything to do with Brotherhood.

" and the ending was more than a little facepalm inducing.  "

   If you're one of the people who had an epic face palm because of the "cwazy stupid space bullshit", stop playing Assassin's Creed cuz that space plot point clearly is essential for the franchise and will expand in future installments, and i cant wait.  

It definetly adds to the story, it's practically a sequel, the other half of the story of what happened afterwards.

Yes, I'll agree with Ryan that they really should have called this Assassin's Creed 2: Brotherhood. It picks up from the moment we leave Ezio in AC2 to the point where if you haven't played AC2 you'll probably have no idea what is going on. So far it seems rather essential to the continuing story as opposed to those throw away DS and PSP games.
 
As for the space aliens being seen as pagan gods and controlling man it might be a theme in sci fi but that doesn't make it bad. Where did you think that the Apple of Eden and such came from? I saw it coming from the point where the "twist" in the first game was revealed.

I never understood why the select few praise AC1 for its plot, it was non existent and there was basicaly no character developement whatsoever in the first one. There was also no mystery in between the entire game like there is with AC2 & Brotherhood. The greatest part of AC1 was the ending with the writings on the wall, and that was it. The video, the codex, the pictures, the audio logs by Subject 16, & the writings in AC2/Brotherhood however outdid the AC1 ending.

It's a tamer version of MGS.   

" Where did you think that the Apple of Eden and such came from? I saw it coming from the point where the "twist" in the first game was revealed. "

" I never understood why the select few praise AC1 for its plot, it was non existent and there was basicaly no character developement whatsoever in the first one. There was also no mystery in between the entire game like there is with AC2 & Brotherhood. The greatest part of AC1 was the ending with the writings on the wall, and that was it. The video, the codex, the pictures, the audio logs by Subject 16, & the writings in AC2/Brotherhood however outdid the AC1 ending. "

" It's a tamer version of MGS.    "

" I never understood why the select few praise AC1 for its plot, it was non existent and there was basicaly no character developement whatsoever in the first one. There was also no mystery in between the entire game like there is with AC2 & Brotherhood. The greatest part of AC1 was the ending with the writings on the wall, and that was it. The video, the codex, the pictures, the audio logs by Subject 16, & the writings in AC2/Brotherhood however outdid the AC1 ending. "

" I never understood why the select few praise AC1 for its plot, it was non existent and there was basicaly no character developement whatsoever in the first one. There was also no mystery in between the entire game like there is with AC2 & Brotherhood. The greatest part of AC1 was the ending with the writings on the wall, and that was it. The video, the codex, the pictures, the audio logs by Subject 16, & the writings in AC2/Brotherhood however outdid the AC1 ending. "

Heh to each their own, AC1 did nothing for me and if i had only played that one instead of 2 i would probably not even care for the series right now AC2 was the real innovative one, It was the one that introduced a lot of the back story. I dragged through the whole AC1 game because of not only its lack of a plot but its lack of one in game unlike in AC2 where finding the videos made me break my neck trying to unlock new areas. AC1 just showed us some friggin writings on the wall. 
 
Only thing i would say about AC1 that was better, was its boss & its ending, but when you consider the video, the puzzles and everything else in AC2/Brotherhood they made up for what AC1 lacked and are by far the superior titles. The way they tied together the worlds famous inventors into the mystery was enough to get me hooked.

  So: Is the story just universe expansion, or does it actually move the plot forward? Is it worth playing through for the story, or can I just read the eventual summary on the internet and get everything I need? "

Probably the most critical thing

Crap, I'm gonna say this thread needs more spoiler warnings. 

Some serious shit happens at the end, but that's all I'm going to say. There's also quite a bit more Desmond.