Oddly, the US gets suddenly a lot more liberal vs. the 1970s, but no where near all the way. Racism is dead, 100%. Gays, who just started their protests en masse in 1969, suddenly have a ton of relatively hugely accepting people, in redneck places, but at the same time, the gays from uptime look at them like "WTF? why are you so degenerate, stop." It stops the AIDS epidemic easily.
The big thing is that everyone is going to be pissed about is the 'coming' Roe v Wade case, and the need to stop it ASAP. Also, gun rights is huge in MAGA country, while not a 1970s republican thing.
A big question is what do they do about reallocation of House seats? The census just happened, so it could be 10 years before seats are reallocated.
Temporary poverty is also fairly large, as a large number of people just lost their life savings, as it was stored in computers. This might get solved, might not.
An important thing to do is figure out which important figures are suddenly lost forever, as a side thing, both ones from the 1970s from the 2020.
Also, most important: with military bases, we suddenly invaded the USSR. That's a big fucking problem. If it doesn't trigger war, it does mean the USSR gets a ton of advanced tech too, so no ROFL stomp.