I believe it's similar to how mass effect 2 works, Different choices you make in fable 2 affect how things will change in fable 3
for example - if you payed or didn't pay for westcliffe to be restored, it will most likely stay how you left it. or there could be more population depending on the wish you make at the end of fable 2
another example: say at the end of the game you choose the money then in the next game you will have all thins money but your faithful companion will not be he will probily be resuracted or some thing wil a power spell or something
Actually it only affects one thing: whether they say "the old hero King" or "the old hero queen" when referring to your parent. That's dependant on what your last played hero on Fable 2's gender was. The economy and the world is uneffected, because, well the hero had 50 years where they could've fixed that, huh? Also, Logan has affected the world since that, too.