So, the long skinny of it is, how do I get the variety back in my metal diet? I tied upping the mineral variety in the world gen, I tried other embarks, other worlds even, and I've got nothing. Please help, the lack of metal variety has thoroughly curbstomped my interest in DF. And that scares me.
You can make the whole world metal if you like. I have done it before - everything was gold, adamantine, and marble

. This was in 31.18 and I had all the other metals too about the place.
I added the [SEDIMENTARY] etc tags to the definition of the metals - presto entire layers of the stuff (with other stuff as deposits). This was overkill though, I'd prefer to make the large deposits Metal, not the whole layer if I do it again. I cut and pasted the defs to the stone_layer file, I don't know if this is strictly necessary though.
Eg hematite has the tag "VEIN:100" try upping the value or changing VEIN to CLUSTER
The basic idea is look at the tags for something like orthoclase or mircocline, which you get a lot of, and copy/paste relevant tags into the things you want more of.
EDIT : Using DFProsector, on normal, I got 100,000 Microcline ad no Hematite. Then I regenned after I changed Hematite to "[ENVIRONMENT:ALL_STONE:CLUSTER:100]" copied from Microcline. The next map had 60000 each of Microcline, Orthoclase and Hematite. Presumably this figure could be tweaked down a bit (not to CLUSTER_SMALL, that's for rare gems only).
Setting Hematite to [ENVIRONMENT:ALL_STONE:CLUSTER:20] plus [ENVIRONMENT:ALL_STONE:VEIN:100] gave about 30000 Hematite to 65000 Microcline.
Actually just increasing the environments they appear in to ALL_STONE should give you a broader mix of metals.