But it's not like people could magically move through each other in the '70s, so why is it slower now? Baggage fees, for one. Fewer people checking bags means a lot more giant carry-ons, meaning a lot more people clogging up the aisles trying to put them away. It's not the only culprit, but it's a big one.
Spirit, of all airlines, has actually found a handy (if onerous) solution to this: charge for carry-on bags, too. (It charges more, in fact, than it charges for checked bags. And if you have to check a bag at the gate, you can get hit with a $100 fee.) Naturally people become more likely to suck it up and check their bags. The airline claims to board about five minutes faster than its competitors.
The Southwest solution
Southwest seems to have figured out at least part of the problem. Its famous open-seating process -- whereby passengers split into three groups, line up numerically, and choose any open seat -- is significantly faster than the back-to-front method. Theoretically this would create a mad dash for seats and an even greater clusterfuck, but it turns out to work smoother than you'd think.
People sit down faster as they board a Southwest plane, and the line moves quicker. Think about it: If you know you prefer an aisle seat, chances are you'll take the first one you find. You stow your bag and get out of the way. If you have a big carry-on, you're more likely to take the seat near some open bin space, rather than look for open space THEN go to your seat.