All the airline sites seemed to have converged to a common model. They all follow exactly the same flow: pick cities and dates, then pricing, then sign in, etc. There was far greater variation in the flow in the early years.
> but from a usability perspective it might just be the worst
Oh, I can cite major airline websites that are much worse from a usability perspective. Trying to book a flight on any Brazilian airline is impossible unless you're Brazilian. That's because every airline in Brazil has inexplicably decided that they want you to enter your Brazilian identity number -- explained in more detail here:
http://brazilsense.com/index.php?title=Booking_a_domestic_fl...
If you're not Brazilian, you don't have such a number of course. Note that this is not any sort of legal requirement; it's just thoughtlessness about usability by the airlines.
I expect a lot of indignation this summer when visitors try to arrange flights inside of Brazil during the World Cup.