Nausea, gagging, and vomit as far as I'm concerned. As was once said -- I don't eat anything that looks like it fell out of a horse's nose.
A very real risk of contracting Hepatitis A if they are undercooked or raw. Aside from that, nothing that you can't gain nutritionally from several other varieties of shellfish.
Folklore gave it status as an aphrodisiac in part due to the shape of an opened oyster resembling certain body parts but scientific studies have proven this to be untrue.
None. Their properties are to be found in other sea food. It is a very bad idea to ever eat a live animal. It just cannot be justified. If oysters made a difference, it would be shouted from the rooftops, and medical research would take an interest and then sell their findings in a big way. That hasn't happened. It is just ridiculous. A food that is said to have properties not found in any other food anywhere in the world apart from the tropical rainforest is the brazil nut. Eating one a day can do wonders for the skin, and I have read it can increase hair growth, but I dont know if that is true or not.
Next time you have a cold, spit out a walnut sized gob of phlegm, refrigerate for 30 minutes, then swallow. If that turns you on, please let me know why???
My father made me eat one once...I was told I had to swallow it whole...it was huge....about the size of a golf ball....and slimy and horrible...pretty much the worst eating experience I ever had, aside from being forced to eat pea soup.