For one, there's all sorts of new species appearing all the time that you can try. Most can be tank bred so you aren't adversely affecting the environment AND you can try breeding the fish yourself (which is extremely rewarding but can also be quite challenging).
Alot of SW species can be tank pred or propagated. The only reason why people dont tend to do it more often is its more expensive than wild caught species, so you have a lower profit margain. You can buy all your stock for the marine aquarium that have been either propagated or captive bred.
With saltwater species the selection is far slimmer.
Sorry Sylvia, but I disagree extremly with this post. Youve either gotta know where to look, IE shops, or have the set-up to support a wider choice of inhabitants once you find them for trade. There are more species in the ocean, and which are readily available in the trade compared to FW. Macroalgae and corals outnumber FW plants etc etc, and I could go on but wont!
Also, I like the fact that I can mix and match fish from different parts of the world or choose to do a biotope, or I can do a single-species tank or go for one that's color coordinated lol!
You can do this in a SW environment also. There is no difference. People can buy whatever is availabe to them, and that means in my case it can come from the great barrier reef, the pacific islands, or local. I could set-up a biotope, infact, that is what you do when you set up a reef. Low nutrients, high lighting, high flow, specific corals, reef compatible fish, and youve got yourself a reef biotope from anywhere in the wolrd if you keep the species correct. There are even SW planted tanks which use macro algae, with compatible species of fish such as seahorse, pipefish and reedfish. You can go colour co-ordinated if you want aswell. You could do a single species tank, a predator tank, a fish only tank, a tank for tangs, a tank for a shark like crazyelece. The list of possibilities is endless.
With marine tanks, things are somewhat more restricted. I suppose though, it does come down to your personal prefferences. If you find a fish you realy like, it doesn't matter what kind of water it lives in
Im confused by what you mean by "restricted"? I what sense are you stating marine tanks are restricted? They arnt. Its what you do to them, what your willing to keep, and what your willing to spend on the tank in question.
IMO, you cant accurately compare saltwater or freshwater tanks until youve done both. It just cant happen until youve actually experienced the upkeep, the species of crustacean, macro algae, bacteria, sponge, worms, the LR, the fish choice, and the corals on offer.
I was like you at first, thinking about it, only dreaming that I could own such a diverse enviroment in a contained aquarium. Then I thought to give it a go, there are ways you can save heaps (doing a FOWLR system is MUCH cheaper then a reef + it's a great way for beginners to start in marine). I have found that species selection for SW is quite varied and there are lots of good nice interactive fish specimens for beginners that you can choose with heaps of personality. If your not too fancy about fish you can always have a small invert tank. Whatever suits your desires. 
Don't hesitate to ask any questions whenever one springs to mind. 
