SW isn't all as hard as most people make it sound. Actually, it's relatively easy if you have the money and patience. Like said...the number one thing is cost. A reef tank is much more hard on the pocket than FO or FOWLR tanks. But innovations nowadays are making tanks more and more cheaper to own.
RO/DI, which is much prefered, isn't neccesary. I only say that beacuse I've used it tap water almost 5 years now with no problems except nitrate issues and some algae. Of course, it isn't always as easy and I'm very lucky with my water. It doesn't contain nitrate or phosphate from the tap like most well water does. My nitrates stay around 20, which is about as high as you want to get. Most SW fish and inverts can handle more than we think they can. Nitrates of 20 are fine for them, but like always, lower is better.
I'd say that anybody who wants to start a reef tank or SW tank in general should, if they have the money. It's expensive...very. I have probably spent around $2,000 on my 29 gallon tank alone. But, after the initial costs of setup, maintaining the tank is relatively easy. A water change twice a month and monitoring water chemistry. Of course, just like FW though, there are varying difficulties depending on the species of animal you wish to keep (high light coral, low light coral, anemones, fish, etc.)
MCD gave some good examples of the differences between FW and SW in regards to filtration, lighting, water movement, etc.