Things You Missed Out On if You Never Watched the 2003 Original Series
The birth of Elicia Hughes
Which canonically happens on Edward’s birthday
Shou and Nina being a much bigger part of Edward and Al’s life.
They weren’t just some family the brothers met once. They lived with the Tucker’ for months before Nina died.
Al being smart enough and advanced enough to pass the first part of the Alchemy Exam, pulling out solely because it would be dangerous for people to know he didn’t have a physical body.
Barry the Chopper as a human. Not only do the brother’s meet him before Laboratory 5, but Edward and Winry are almost killed by him.
By far the most iconic of any of the FMA/FMA:B openings
Hughes being crushed by a 12 ft monument of Elicia
Roy and Edward head to head in one of the most epic duels of all time
Al and Martel being really close friends
Lust’s character being 176% more important and compelling.
Like… way more important. I personally like the homunculi creation system in the original a lot more than in Brotherhood. With plotlines like that being so different there is no objective ‘better’. But, the way she was created paved the way for not only one of the best character study episodes (35, ‘Reunion of the Fallen’) but one of the best character redemption arcs, as it’s fueled by revelation of character motivation, not a moral change of heart.
Speaking of: the dynamic/relationship between Scar and Lust
I was genuinely shocked when I first came on Tumblr after finishing the original (before starting Brotherhood) and found absolutely nothing shipping wise between the two. I know now it’s because the majority of the fandom is dedicated to Brotherhood, but still. Maybe it’s just me, but I found their relationship to be one of the most complex and fascinating dynamics in the entire series.
TBH, I think that with the exception of Selim and Greed (and maybe Bradley), the Homunculi in general are way more fleshed out, sympathetic, and so much more intriguing than just the evil bad guy henchmen of Father.
The Flame Alchemist, the Bachelor Lieutenant and the Mystery of Warehouse 13
One of the most iconic Royai scenes in history
Sheska being a much more important character and her being a key player in solving Hughes murder
SHESKA AND WINRY BEING BEST FRIENDS
This is all I’ve got right now just going off the top of my head but I’m sure there are way more.