1. On the river, the cast matters zero; on the flats the cast is paramount.
2. On the flats, your fly pattern selection counts about 1/10th what it does on the river or lake.
3. You fight saltwater fish with a low rod angle, and put the pressure in the line/reel; pull that stunt in freshwater and you're done.
4. On the flats, there are fish that can eat you; on the river, there are rapids that can eat you.
5. On the river, you want those flies to dead-drift toward your target; in the salt, the fish aren't used to their food attacking them.