So, you're saying there is a finite limit on available iron?
When you produce a unit which requires Iron (eg. Catapult, Swordsman) it uses up one unit of Iron. This produces a limit on how many resource-dependent units you can produce.
I like it as well, but they need to make all iroon the same amount. My current game, while attacking Liz, a barb killed my mine and my units died in battle. They can't take a hit at all, and don't do much damage either.
Last game I started on a small island with 3 neighbors. I had one iron value 2, while my 3 warring neighbors all had 1 iron each and all value 6. needless to say I got whooped.
Though keep in mind that the Strategic Resource is considered unused when the previously using unit is killed/deleted. So if you have 3 Iron, Make 1 Swordsman and then have 2 Iron. Once that Swordsman is removed from your control, you will return to 3 Iron. Resources in Civ 5 are not like most games, where you gather more over time by mining. In Civ 5 when you build a Mine on a resource you immediately gain control of that particular quantity of the resource.