while i'm no expert, i think you'll find that salt water rods are much stiffer and faster action-a big plus when casting in the wind or when fighting bigger fish. fresh water rods, especially for trout, steelhead or salmon are usually not as fast, and since you're usually drifting a wet fly, the cast and the type of line is quite different and mending must be considered, so a longer rod is in order. the presentation is usually much lighter than in salt water (bones and permit notwithstanding). dry fly presentation is softer yet.
by way of example, i purchased a 9wt 8' 3 piece crosscurrent for peacock bass fishing in the amazon. it's a salt water rod which gloomis recommends for peacocks. it is the best peacock bass rod i've come across: stiff as a broom handle, which is great for ferocious strikes and for holding the big mean brutes away from the mangroves. i now use it exclusively on my boat for stripers/bluefish. the shorter length is easier to control on a boat and it casts a mile and cuts through win with ease. that said, it would be terrible for steelhead or salmon. the stiffness would work against delicate presentations, and the short length would all but eliminate mending. it is probably so stiff that the light tippet would likely break if a good fish strikes hard-no give in the tip.