It's gotta be the use of bell-bottom pants - something about the way the extra fabric absorbed the reflections off the real hardwood floors.
Seriously though, it required a
serious investment of time and money to produce a record back then. If you were an amateur, you probably couldn't afford it. If you were a pro, you had to be one of the best of them (and under the tutelage of a top-notch one) to even touch the board. Most producers started as engineers; most engineers started as assistant engineers; most assistant engineers started as tape ops; most tape ops started as janitors. You had to earn your stripes.
Another point (made earlier but worth reiterating): there were a lot of crappy recordings back then, too. It just so happens that only the good ones are remembered or talked about now, hence the false impression that
all recording was better then.