Wish List for ideal website features
's booking system - Let's you see cheapest fare available not only on day you want to fly but up to a fortnight either side to find cheapest fare. Great for finding ideal cost/flight time options cheaply, with benefit of doing each leg separately to get combined total that cheapest/most convenient.
's seat reservation system - See what seats are available to pre-reserve when you book, with option to change as required.
's seat maps - full aircraft with galley and door locations (Not just one long line of seats)
's timetable - Shows for day you request, plus three days either side (a week!), showing direct and connecting flights, and scheduled aircraft.
/CO online check-in - Not tried any other airlines for this but these two both easy to use. Personal preference for BA
, but only because no seats are reserved exclusively for frequent flier memebers.
Can go on but those four combined would make a pretty good reservation system online.