Salt water isn't really "harder" so to speak, but it can be a lot more time consuming, and it's a whole other world if you have only dealt with fresh water. Mostly, like many said, it's the money, but also, there is more chemistry involved, and so on. I have spent thousands of dollars over time. On my smaller tank (55gal), I spent over 2,400 in equip., lighting, supplements, livestock, corals, rocks, and sand. (now granted, I had to replace a few fish from Ich) so minus about $150, and that's how much money (and even more!) that you can end up spending - if your obssesed like me, lol, if not, you can opt for cheaper lighting (though I shouldn't even be allowed to use the word "cheap", as they are NOT),and less expensive fish and corals. Either way, I love it, and can't seem to stop!
--EMI