In G&K now, compare:
Aluminum is the most important resource in the game: it's used in Modern Armor, Jets, Stealth Bombers, Hydro-plants, Spaceship Parts, Helicopters, and Spaceship Factories. Hence, it gets both Recycling Centers, as well as larger standard yields (8 vs 7 coal, 8 vs 4 plutonium, etc.).
Coal is the least important. All you need it for is Ironclads and Factories, though factories can be especially important in a few cities
Plutonium is in the middle. Solar plants can now replace Nuclear Plants in some situations, or, more typically, just for a city or two. But nuclear weapons are very important, with the three of them (Atom Bomb, Nuke, and GDR) representing the most powerful units in the game.
Oil is also in the middle. While oil is not used in a single building, it is very important for those earlier air units, as well as landships, tanks, and battleships.
Now, compare Iron:
1) Iron is not used in a single building (although, improving an iron tile allows access to the Forge in that city)
2) Iron now only supports Swordsmen, Longswordsmen, and Frigates, with Swordsmen being nerfed in G&K compared to pikes. So, really, two viable units to build.
3) Iron tiles have quite low yields (if all you get is a 2 iron tile, and you're Japan or England, this actually is a problem, though)
In other words,
Iron is used in zero buildings, which is a trait shared only with Horses and Oil.
Iron only supports 2 standard units worth using, and longswords are now much more quickly obsoleted by muskets. Thus, Iron is the least useful resource for units across all resources.
Iron thus has a very small amount sprinkled across the map, often times.
This seems to me like trying to apply two fixes to one problem: in vanilla, Iron was far more useful for units (swordsmen were great, early siege units needed iron, and so forth), but now we have not only fewer units using iron, but also one of the primary reasons to rush Iron (swordsmen) being nerfed by a unit that appears in a better tech path (pikes + irrigation bonus = I'm going Civil Service most games now before touching the lower tech path).
Compared to the other resources, Iron went from being incredibly useful in vanilla, to being almost a throw-away in G&K. I could understand nerfing swordsmen, or reducing the number of early game units needing iron, but not both. And frigates needing iron is not good compromise. I mean, if I need iron for frigates, I'm usually at a point where back-tracking to get Iron is not a huge deal: in other words, if the best reason to rush Iron is for frigates, which appear a lot later, then there's no need to rush at all.