This is fine as a feat or technique. I would just warn about making them too specific in terms of having the player asking a lot of questions and thus slowing down play. Might be hard to reward something and make it cool vs slowing down play. Say you have a cool axe power that gives a +1 to hit vs chain mail or takes away half the AC bonus from scale mail. Then the player is asking if every monster is wearing chain, or you need to figure out what the new AC would be.
This is why I thought of the 10ft teleport. It puts the power on the player to use when needed.
