You are right about there being a point where you can over marinate a steak. However it has nothing to do with the flavor imparted to it by the marinade. That is a personal taste preferance, and i find it hard to tell someone that their steak is over marinated due to being able to taste very little of the meat. I have had some teriyaki/and other asian marinated dishes which had very little taste of the beef left behind, but they were fantastic.
You over marinated your steak when the texture of the cut you are using suffers as a result of the marinating proccess. A marinade contains three things Acid(s), Aromatic(s), and Oil(s). The issue is with the Acids. The Acids will cook the meet/poultry/seafood, very slowly. But, in the process the item becomes more tenderized. You marinated your item too long when your item starts to get the texture of a gum. For beef this takes more than a day to occur. For poultry/seafood it takes less and less time. It has nothing to do with the taste >.>
You do not always cook steaks at low heat. That is just as noob as always cooking steaks at high heat. A good rule of thumb, is that the parts of the cout that actually move need to be cooked long and low (barbequed or stewed), or treated with a marinade to help tenderize the meat before hand. On the flip side, is the portions of the cow which do not move often, are best grilled on high heat. If one of these cuts is especially thick, you start the proccess grilling on high heat, then finish cooking in a low temperature oven to prevent burning before the internal temperature reaches the desired point.
I am sure that you enjoy the way you grill, but low and slow is not the way to cook everything. It depends on the cut/grade/and thickness of the meat.
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