Unfortunately, time hasn't been very kind to Fullmetal Alchemist, critically anyway, as the appreciation for its novelty has diminished in favor of Brotherhood's execution. But, this series still stands as an enduring classic of the past decade, and in my opinion, of all time.
As just previously mentioned, Fullmetal Alchemist is now a decade old. You wouldn't necessarily notice it though. The technical aspects of the show are still excellent, even for contemporary standards. The art is slightly less lustrous and lacks some of the detail exhibited in current anime, but that's really where its age stops showing.
Fullmetal Alchemist is one of the finest animated shows you're likely to find. Bar none. This is one area that cannot be stressed enough. I'd go so far as to say the action set-pieces in Fullmetal Alchemist are animated better than Brotherhood's. That's no slight either, this show really is a visual treat.
The quality of the audio, on the other hand, is more of a subjective issue. Most have their preference on dub vs. sub, but don't worry about the former being particularly inferior to the latter. The dub is very good, and stands equal to its Japanese counterpart. The soundtrack has a few memorable pieces, such as "Brothers" or "Beaming Sunlight" which are poignant enough to wet most eyes. The series also has four openings and endings. Each opening is initiated by an almost melancholic monologue by Al and, and later Ed, speaking in a manner that implies they're reflecting on the events of the series. An interesting notion for sure.
That aside, as most are aware, Fullmetal Alchemist veers from the plot of the manga. It's essentially an extrapolation, but one that isn't far in accuracy nor a significant downgrade in quality. The first 10-15 episodes seem largely unrelated to the main plot, but afterwards, the plot quickens and tightens until it reaches a conclusion that's equal parts satisfying and frustrating but ultimately, justifiable.
Another important distinction is the story's tone. Fullmetal Alchemist is a more mature tale than the source material. Be forewarned, characters in the manga who live, face more depressing fates in this version.
But violence alone doesn't make a story more mature, just more explicit. The story is mature because of what it doesn't show, rather than just what it does, and in turn, the questions it asks with the knowledge there are no easy answers. More simply put, the same introspection Fullmetal Alchemist subjects its characters to, is the same kind it expects from viewers.
And speaking of the characters, they're the limelight of the series. Most of the major characters are the same, although increased attention is given to Shou Tucker and Rose, who play more vital roles in the story. The primary difference in the characters, between the manga and the show, lies in their characterization, specifically the Homunculus, Mustang, Scar and Ed.
This is one area where Fullmetal Alchemist usurps its source. The Homunculus are considerably more humanized in this version, which makes them easier to sympathize with. They're more independent, more nuanced and more complex overall. They aren't just a physical manifestation of a sin. Rather, they're a contrasting mess of old desires, memories, and emotions resurrected and personified. They're just as much victims as they are aggressors.
As are Mustang and Scar. Both are men haunted by their pasts and carry large amounts of guilt. Mustang is a more fragile character and Scar, a younger and more emotional one. As the series progresses, both increasingly look to finally rid themselves of their respective burdens.
Lastly, there's Edward, who is less naïve and less of an idealist than his manga counterpart. Even so, Edward's staunch belief in the philosophy of equivalency stays adamant until the end, when he learns from both Dante and from the Gate, that there isn't a universal way to quantify fairness. In some ways, despite all he goes through, Edward is the one main character who changes the least. But to his credit, he is called Fullmetal.
I suppose I wrote all this to get one point across: Fullmetal Alchemist is a masterpiece. Nothing else really needs to be said.
