there are three fundamental oysters on the US oyster scene: crassostrea virginica, crassostrea gigas and crassostrea sikamea. they are the east coast oyster, the west coast oyster, and the kumamoto oyster.
we rarely see belon/flat/european oysters, or any of the wonderful little critters that are locally specific to europe or the rest of the world. we see oysters that are either locally abundant or easy to farm. the east coast oyster is really the only native oyster we see in quantity, the gigas and sikamea oysters are both indiginous to japan. a caveat would concede that the west coast "olympia" oyster is also a native species, and is a fourth species that we would see occasionally (seasonally). they are about the size of a nickel and have a unique mustard/caper taste.
All of the "named varieties" of oysters are simply locality specific. A Fanny bay oyster is a west coast oyster (gigas) that comes from (duh) fanny bay...but so does a bay shore oyster...basicly the oyster company picks the name they market with. the flavor differences between oysters of the same variety are rarely profound; a california raised gigas oyster will taste different than an Alaska version of the exact same oyster, but you might have trouble telling the raw meat of one oregon oyster from another. its not that the differences arent there, they just arent really big or easy to quantify.
the variation based on locality is derived from a few different things: water temperature, available food/type of food, salinity, and rearing method.
an oyster from cold water will be sweeter than a warm water counterpart, this is because oysters store emergency energy/fat as a form of sugar. a coldwater oyster stores more emergency energy by percentage for the hard times ahead, so to speak.
usually oyster farms are located where the locally abundant flora can support the oysters. if one group of plankton is more prevelant at one location than another, the oysters are going to taste different. if you feed a pig nothing but delicious acorns, it will taste different than a pig that eats nothing but grass.
the ocean and rivers that oysters are raised in vary in salinity. this is pretty straightforward, i havent noticed a big difference in other flavors based on level of salt in an oyster.
last, the amount of time and method used to rear oysters effects the shell depth, strength, size, and shape. there are several techniques that oyster farms use to effect these variables, most of them regard changing the oyster's depth in the water column depending on its age or time to harvest. the term "beaching" is one method of bringing oysters into the surf from deep water so as to build a more durable shell.
all that said, the gulf coast oyster is a big, fleshy, relatively mild oyster with a flavor probably best described as meaty. some can be eaten with a knife and fork. the gulf coast oyster is crassostrea virginica, the same oyster as the famous blue point. they are definately the fowl of oysters; not particularly flavorful, but great for some things. i can think of few things more sublime than oyster stew or fried oysters, and i would use no other oyster than the east coast oyster for that.
as far as health is concerned, any warmwater "southern" oyster is more likely to contain potentially dangerous level of the oyster bacterium vibrio vulnificans than a colder water northern oyster; which can be fatal, although usually just makes people really sick. The state of California has banned the sale of gulf coast oysters within its borders due to this bacterial problem. i do not know of any other contaminant problems in legally harvested or raised oysters.
as far as my opinion goes, most people are unlikely to get sick from eating oysters. i have eaten probably several thousand in my years in the industry and have never gotten sick. yes, i throw out the stinky dead rotten ones, but thats a whole different dangerous bacteria that causes that. the oyster fishery in the gulf coast is important to the local economies and the gulf coast oyster is pretty tasty. i say, eat more oysters.