Well the usual horror of live action (imo) is that the anime characters tend to look pretty damn unrealistic with all that spikey/freakishly long hair in strange colors and the clothes they sometimes wear..it just looks laughable on real people.
In FMA they don't have that problem, though (besides maybe Envy). There are enough blonds out there and the clothes are pretty normal. Plus the style of the anime/manga was never all sexual or bishie-style, so that's no problem either.
And there are no
terribly unrealistic karate-move-things like in Naruto or wherever. So that should be kinda fine as well. Just tune down on the manga-expressions and exaggerations and everything's good.
Then again, the plot and special effects, as well as setting and all that require some money to actually look good and if they attempt some half-assed movie...then there's a lot of cringing that will ensue.
