Originally posted by Rage.Of.Olympus
It was sort of an unspoken rule until JMS came along and he had Balder point it out. Being on Midgard doesn't diminish Thor or Loki however. In regards to Odin, I'd put him in the same boat as his sons on a daily basis as he possesses his own internal reserves.
I'd say even Thor is weaker on Midgard then he is in the other nine realms. Brunhilda ran him through the heart with a spear in Nidavellir (I think), and it didn't even slow him down.