Steven Rosswurm’s book, The FBI and The Catholic Church, details the Hoover era FBI's efforts to obtain Catholic moles and informants. Remember that until after WWII and it was showed how they joined and fought en masse, Catholics were never part of the WASP mainstream in America.
The FBI's effort was somewhat of a success, in that Communism is antithetical to Catholicism. The result was a theology that made "the enemy of America became the enemy of the Church" when in fact America was the enemy of the Church. Later, the feds would bribe bishops with government grants.
After WWII, the ethnic enclaves of Poles, Italians, Irish, etc. were broken up by the importation of southern Blacks into northern cities and the "urban renewal" projects that put low income public housing projects into those ethnic neighborhoods. Catholics moved to the suburbs, and became atomized consumerist Americans.
After internal changes to the Church, the average Catholic was attending St. McSurburbia with a watered down liturgy. Many left the faith, and/or didn't raise their kids Catholic.
In short, there are far fewer Catholics today. While there is a progressive wing of Catholics today, they are fewer and diminishing, and the trads are in the process of retaking what was lost in the 1960s/1970s as the older generation dies out and their kids never joined the faith.
Aside from the whole bit about the authority of the Pope, as a matter of doctrine, I believe there are only a few things. There are a couple big ones; Orthodoxy allows for easier divorce and for contraception (not that most Catholics follow that prohibition, sadly).
At one time, there was a big controversy about whether (in short) the Holy Spirit descends from the Father and the Son or just the Father. But I'm told that properly understood, that this isn't really a distinction that matters. I'm no theologian though.
I have nothing but respect for Orthodoxy, who take much greater reverence in the Eucharist than most Catholics, and further, stick to the old rules for fasting, etc. and not the watered down version that Catholics use today.