I know he was nominated for the Oscar, but didn't win. Did all critics love it, or did many dislike it greatly because of how new and untheatrical it was? Also, how long did it take before other actors started imitating his more natural style?
There were many other naturalistic actors at that time, and for a long time before. Many of them were every bit as good or better than Brando. But the "pre-method/post-method" myth will never die.
Correct. Brando was considered to be brilliant and revolutionary, and for a long time had a monumentally inflated reputation. I remember when I was a kid in the 1970s, and his "Mutiny on the Bounty" came on TV, and some relatives came over to watch it and all the adults told me that Brando was the best actor in the whole world! Which kind of killed the movie for me, nothing keeps a person from enjoying something like too-high expectations.
So no, the conservative old farts at the AMPAS didn't take to Brando for a long time (and regretted it when they did), but their reluctance didn't stop Brando from becoming immensely popular with the public and influential with his peers. He really was regarded as the actor's actor of Hollywood for a while, until he'd been around for a while and everyone had a chance to see some of his worse performances. Frankly, I think that although he could be damn good in the right role, his reputation is largely due to his personal magnetism, star quality, or sex appeal. Nobody knows where that kind of charisma comes from, but you sure as hell don't learn it at the Actor's Studio.
And yet Brando's performance in "Mutiny on the Bounty" is exceptional.
While his charisma partly explains his success, practically all Hollywood celebs have charisma or they wouldn't be "stars" in the first place.
Marlon explained in his autobiography what gave him the edge: He said he noticed actors would blow all kinds of energy memorizing their lines off camera, pacing around and such; when they finally performed for the camera they had lost their mojo. So Brando would just memorize the gist of his lines and have line cards taped here or there to help him out. Thus he saved his energy for the most important moment -- when he was in front of the camera. That plus his ad-libbed delivery gave him an authentic vibe as opposed to mechanically rehearsed.
IMHO the cue card business was part of Brando's general laziness and lack of respect for his craft. And I think the laziness and disdain for the profession was the reason he became a worse actor with age, while his peers who could be arsed to work on their craft and learn their fucking lines got better.
Of course at a certain age, the cue cards were probably there because his mind wasn't what it had been and he couldn't have memorized his lines if he'd tried, and he's not the only actor who used cue cards because they can't or won't memorize lines. John Barrymore did that when the alcohol turned his brain into a dish sponge, and well well well, rumored end-stage alcoholic Johnny Depp is doing it now.
Edit: I haven't seen his "MotB" as an adult, and I don't know that I'd appreciate it now. If high expectations ruined my first viewing of the film, it's likely that my dislike of Brando would ruin another viewing now.