Apart from what has already been mentioned :
The majority of your airport becomes landside, i.e. unsecured, if the checks are only performed at the gates. This increases the chances of a security breach or attack, and also the number of people and thus the space required as you will get non-flyers taking up space.
Ticketing and passenger assistance points need to be centralised too, otherwise the number of trained staff and computers terminals needs to be multiplied.
So whilst Adam is right about the need to change things, what we currently have is probably the least bad. Where I feel a big change is needed is in the boarding process which seems hugely time-wasting, but again having seen various alternatives in use I suspect that there is no magic bullet.