They're re-doing FMA so that they can animate the story as it happened in the manga. With the first FMA series, the anime caught up to the manga by volume 7 (which was around 30 episodes or so. Slightly after the first fight with Greed). While the creators of the anime worked closely with Hiromu Arakawa to ensure that the show went in the direction she wanted, it was still all original animation.

Now, with FMA Brotherhood, they're re-animating it according to the manga, which is going on volume 24 in Japan (in the US, vol. 21 was just released earlier this month). That being said, the beginning seemed the same because it was already animated in the first series, but (1)They cover the material that was already animated in a more abridged fashion and (2)That being said, the series starts to diverge from the original anime around episode...14 or so? (I don't remember the exact episode, but still--it's when they fight Greed).

So yeah, this one's going according to the manga, so, if you're familiar with the manga, you'll get to see the characters from Xing, the differences in the Homunculi, the Ft. Briggs troops, etc.