Best I can tell there are two general differences in freshies and salties. As myfishcasting mentioned, there's the hardware, which is a big deal, trust me. Also, saltwater rods are often expected to be used on bigger, tougher fish, so they'll have little extras like fighting butts to help.
The other difference is rod action. Most saltwater sticks are a faster action than freshies. Saltwater fishing often takes place in windy conditions where long casts and heavy flies are called for. Faster action rods generally generate more linespeed and (all things being equal) will throw bigger flies farther into the wind. However, lots of your high-end sticks (and some of the lower ones now) are sort of anthing goes. Take the Winston's BIIx for example. In smaller sizes it's a great trout rod. It loads easy, casts a smooth loop, and, when you need it, the butt end can bomb a big streamer out there. In bigger sizes it's a great bonefish rod and I've also heard of folks using them for tarpon in the 12-weight size. The soft tip makes short presentations easy but the big butt-section can really put the heat on fish.
So, if it were me and I wanted to do both fresh and salt I'd buy a rod I liked to cast that had the appropriate saltwater hardware.
Hope that helps.