Shounen anime tend to be like Kraft cheese singles; they taste delicious when you are young, but as you get older you realize they are actually awful and have too much filler. I kind of miss the days when I could watch Naruto and enjoy it, but there are only so many meandering, waste-of-time 100 episode wankoffs that a person can reasonably enjoy before they become completely jaded. Have you read my sappy, cloying review of One Piece? I actually hate that show now.
Given this, Fullmetal Alchemist: Brotherhood is something of a miracle. Unlike 99% of shounen anime (including the original Fullmetal Alchemist), this series just doesn't fuck around. FMA:B almost relentlessly pursues its (amazing!) plot, only briefly pausing for when characterization is needed.
Many anime these days are criticized for being "dark" but relatively brainless. FMA:B bucks this trend nicely by having a story that is not only thoroughly entertaining on a visceral level, but thought-provoking as well. There are two major themes at work here. The first is apparent, with the anime all but telling it to you episode one: the concept that nothing in life comes for free; that every perceived gain requires a sacrifice of some kind. A leap of faith is required here; throughout the show, characters appear to be ignoring this stated rule, willfully violating the laws of entropy. But taken at arm's length, this is a powerful theme that drives the show's narrative. There is an underlying sense that the forces of idealism are being weakened, attacked, and even defeated by the unfeeling laws of the universe. The second theme is less obvious, but equally potent. It deals with human hubris; the idea that mankind overestimates its ability and worth in the universe. Striving to obtain more power than we deserve, we pay a terrible price in the name of progress.
The series isn't perfect, but complaints that come to mind (a bad first episode, some annoying side characters, bad OP/EDs) are pretty much nitpicks in what is as a whole a remarkably consistent and rewarding watch. It says a lot that I couldn't stand the original series (it suffers from the same filler overload of a typical shounen show), but loved this one. All told, FMA:B is perhaps the closest thing to a series I would literally recommend to every anime fan.