About mining, i am not sure, maybe you could interprete it as really digging the interesting mineral and not just the "normal" stone, but i don't really know.
About Metallurgy: I think it is right that after some point you will not learn anything new from very simple works, maybe it could be a little more gradual, so after some time you get less experience and later none, but that is not too important i think.
But your comparison to fighting rats is good. Fighting easy enemies should get less rewarding when you get stronger. By now this was addressed like in Hack 'n Slash games: The higher your level is, the more EXP you will need for the next level, so fighting easy enemies is no longer rewarding. But with the possibility to get EXP from different sources than only fighting, it could be good to have a formula to decide how many EXP you get, but i don't know how it should look like because your fighting strength depends on several stats and skills and your weapons.
Or even better: we get a system without EXP where you get specific experience in what you are doing, and nothing else, as discussed several times (one example:
http://hydlaa.com/smf/index.php?topic=24199.0 but there should be more threads like this out there).
