I just started my first sw and its a 10 gallon nano. I guess the biggest differences between sw and fw, without the obvious salt are:
You can not keep nearly the livestock in sw as you can in fw
Water conditions must be near perfect if not perfect
more expensive
However, it's so cool to just look at the live rock and what comes on it. I just got a clean up crew and thats cool. Now I am gonna wait a bit and add a couple fish and thats about the extent of livestock. Now as far as corals and anemones go, I haven't gotten that far yet, but they need alot of lighting and most can't be kept in a 10 gallon. But it has been a rewarding experience thus far.