Well, technically speaking you CAN trade food now, just not 'food units'. You could choose to trade wheat for corn, or beef for sheep or something along those lines. The idea is that every additional source of food a civ has access to means greater health benefits for the nation-which indirectly impacts on the growth of their cities. So, not only will food trade be important, it will be much MORE important than in Civ2 in many ways.
On the issue of Economic Victory, I too would love to see such a thing-and wonder if it can be modded in. Perhaps something a little more detailed than the 80% suggestion, though, as that may end up becoming little more than an excuse for war and conquest.
How about this for an idea-each basic trade in a good gains you X point per turn. Getting 'monopoly' access to trade with a particular civ gets you double the normal number of points per turn. Each additional deposit of a resource to which you gain access-through non-violent means-gains you additional points per turn. Each nation you have open-borders agreements with also gain you points, as do the number of Merchants and commercial improvements.
Lastly, having either a Mercantilist or Free-Market economy would double all of the points you gain each turn. The player who gets X,000 points first wins the Economic victory. How does that sound?
Yours,
Aussie_Lurker.