One difference between the old and new settler systems is that it's no longer so much of a disadvantage to have large cities build settlers. In CivIII, the higher the population rose, the more food would be necessary to generate a new population point, so a city with two population and two extra food/turn would grow more quickly than a city with ten population and the same extra food. Clearly, it would be more effective to build settlers in small cities, where the population lost would return much more quickly.
This also means that city populations are much more stable, which helps eliminate some micromanagement issues.
(edit) I think this change also makes it possible for a stable-population city to produce settlers. In the previous game, if you had the bad luck of starting in a food-poor region, you could produce settlers and workers only with great difficulty.