Reflecting on things a bit more in terms of gameplay I really can't see a problem by allowing food to be shared on a national level. If you really want to use all your farms to support a size 70+ city, that's fine, it will just mean that your other cities are smaller. In addition, the health and happiness systems already put a soft cap on population size.
I wonder whether there's any point in disagreeing here, because the factors you quote as limiting the reasons why I would see this as a problem are based on things I also see as problems with Civ IV.
Using all your food to support a size 70+ city could become unbalancing depending on an awful lot of other factors in the game, anything that effects the equivalent of
a Civ 4 legendary city, a Civ 3 20k culture victory city, or a Civ 2 Super Science City. It depends on the synergies, really. If there are ways in which building various wonders and improvements in the same place gives a synergistic effect greater than building them separately [ for example, building the Colossus to give 50% extra trade, then in the same city building Copernicus' Observatory to give an extra 50% boost to science atop that, being significantly more useful in terms of improved science performance than building them in different cities ] then any mechanic that supports building a big city fast makes a big difference; it's not so much the 70+ city that really seems unbalancing there, as the city that gets to size 22 while everyone else is still scrabbling to get to size 4 or 5.
If it really becomes a problem you can always just throw a hard cap on city size.
I'm not in favour of an absolute hard cap on city size, but different hard caps that need specific buildings to get past are another aspect of pre-Civ IV versions of Civ that i would like to see back in the game.
Hammers are a bit more problematic. I'm not entirely convinced that sharing all hammers on a national level is a good idea. For starters it makes wonders problematic. More importantly I could see that the "winning" strategy would just be to dump all your hammers into one city every turn, this way you get to build something immediately.
Which is one reason why I favour transferring productivity via units, which it takes time and commitment of resources to build, and then time to move to where you want them. It becomes an effort to do this, both in terms of where you direct your productivity and in terms of how much work the player has to do, which seems appropriate for getting the reward.