So if you are following Spider-Mans event "To The Ends of the Earth" you have seen the fight that the Avengers had with the Sinister Six. During this fight Thor was impaled in the back by Rhino (so sad) who had the Midgard Serpent Tooth attached to his head. Was it a cheapshot? Yes,. Could Rhino defeat Thor without this tooth? No. It did however show Thor's lack of reaction speed (Rhino yelled at him before he ran into him) and also showed us that all it takes is a being with Rhino's strength with a tooth of the Midgard Serpent to shove it all the way through Thor's body and render him immediatley unconscious and possiblly seriously wounded (though we all know he will eventually recover somehow).
I know a lot about Thor but I was not aware that the tooth is something that could easily slice through his body and have such devastating affects on him. It is however somewhat long overdue however. Thor has been able to run around the Marvel Universe with little direct weaknesses like other heavy hitters have.