For such a vague end speculative question it is hardly surprising that Michael and I should differ in some respects in our replies. For a start, I do not believe that a planet such as Earth is likely to be deficient in iron; meaning that rocky remnants of supernova explosions are likely to be iron-rich.
But, very well, it is theoretically possible even for such a planet to have an iron core, but very little iron in the crust. Whether it is plausible that there would be too little iron to support biochemistry resembling ours, is another matter, but at least such a planet with a molten iron core could plausibly have a decent magnetosphere.
This said, I do not believe for one moment that on a rocky planet with an atmosphere as thick as ours, our magnetosphere is of much radical biological significance at all. It is a navigational convenience for many species, and conceivably some would die out in its absence, but I doubt that such effects would be common. As for its importance in protection from extra-planetary radiation and particles, that sounds to me like so much hooey. I even am sceptical about its robbing us of a significant atmosphere.
Now, suppose some wizard suddenly robbed our planet of practically all its iron less than 10 km deep, the consequences would be extreme and disastrous. All higher forms of life (by "higher", I do not mean everything from soccer-fans up, but everything from most bacteria up) would die very rapidly indeed. Practically all evolution more recent than say, our first billion years would be to do again, and I cannot promise that it would work at all without iron.
In short, like it would be a serious bummer man, with like no one to care.
On the other hand, if a large variety of other transition elements were available (this is implausible on a planet with out much iron, but still never mind that) it is quite conceivable that a range of elements such as vanadium, chrome, manganese, cobalt, nickel, copper, zinc, ruthenium, molybdenum, tungsten, or palladium could serve similar functions, and perhaps a lot of others as well.
This is the purest thumb-suck, please note. It is quite possible that the reason life on our planet is so iron-dependent, is not that it is present in our crust, but rather that it is so plentiful and in fact ubiquitous. That being so, as I suspect, it is likely that those other elements could only have filled such universal roles if they too had occurred universally on the planet, and in suitable forms at that. Not all of them are very active enough under many circumstances to represent promising prospects.
Even assuming that on a planet starting from scratch life could become established on a biochemistry indifferent to iron, but rich in other transition elements, it does not follow that such a biochemistry would closely resemble our own. In fact it does not even follow that life on other, more typically iron-rich planets, the biochemistry closely resembles our own. It might very well, superficially anyway, and working on identical basic principles, but in a matter like this the devil in the details would have an embarrassment of choice.
Such considerations are part of the reason why I regard most science-fiction, and all science-fiction films that I have seen so far, with considerable contempt for their level of either creativity or competence, let alone sophistication in the alien life-forms they present.
