Having not one more carry-on/roll-aboard than will fit in the overhead compartments is easily the most important thing to get right. Making the passengers pay for that space is the key.
This also explains why many airlines do the boarding group thing: because it rewards the frequent flyers with access to overhead bin space!
That's the reason that people want to board first!
So why not just charge for overhead bin space? Because it's unseemly and -anyways- hard to get right.
I flew Aer Lingus recently and the flight had exactly that: 1 free item of checked luggage, pay for hand luggage (except an under-the-seat-in-front-of-you personal item).
It worked great except that the bag-check process was absolutely terrible: every passenger had to go to the desk outside security. But other airlines have solved that issue (separate check-in and self-service-tag-printing+bag-drop-only queues make a big difference).
This also explains why many airlines do the boarding group thing: because it rewards the frequent flyers with access to overhead bin space!
That's the reason that people want to board first!
So why not just charge for overhead bin space? Because it's unseemly and -anyways- hard to get right.