1) Boarding would be faster because fewer people would carry on bags. No more waiting in the aisle for someone to try to fit an overstuffed rollaboard in the overhead, usually 10 to 20 seats forward of their own assigned seat. You also avoid the delays that come with finally having to check carry-on bags because they are too large or the overheads are full.
2) Deplanning would be faster, and safer. No more getting thumped on the head from either shifting luggage falling from bin when it is first opened, or someone not strong enough to remove the luggage without it falling on you. You also wouldn't have to wait while a fellow passenger tries to pry the same overstuffed rollaboard from the bin.
3) Flight attendants would have better attitudes knowing they wouldn't have to deal with surly passengers who insist on their steamer trunk should fit in the overhead bin. Also, they would be less likely to hurt themselves while trying lift a bag that could weigh half as much as they do.
4) Interiors of planes would be more open, less claustrophobic because the bins wouldn't be inches above your head and extend nearly to the middle of the cabin. Compare the interior of the B-707 with that of the B-757.
Photo © Ellis M. Chernoff
Photo © Daniel Werner
5) Getting through security would be faster because there would be less bags to scan and search.
Certainly the airlines would have to do a better job of handling checked baggage, but if you ask me, I'd prefer smaller overhead bins.