Welcome to the marine side of things! I keep FW tanks as well. Much of the knowledge carries over, but some concepts are a bit different.
You already learned about the importance of RODI. Other things that are different (off the top of my head):
1) cost of equipment - tends to be much higher for reef tanks. $300 on lights is actually not bad! But you can definitely keep less light demanding corals without the crazy expensive lights. A good protein skimmer can be expensive too. Or a sump set up. You can't just pop over to petsmart and buy a $30 filter and call it good. So the start up costs can be a lot higher.
2) think of any marine tank the same way you would for a FW setup housing a delicate, wild caught species that is very, very particular about its water parameters and feeding. The vast majority of marine fish have not been captive bred for generations like many FW fish have. So they tend to be much less forgiving of wobbles in water quality or conditions that don't suit them.
3) all the bad stuff happens a lot faster. I could write a novel but I think that sums it up. Parameters change, tanks crash, algae overtakes everything...just a lot quicker than it does in FW.
4) much greater risk of disease, due to almost all wild caught livestock. QT is really, really important. You can't treat for a disease IN a reef tank (the copper will kill everything and can be absorbed into the live rock) so if you don't QT you may have to be catching fish in a reef tank full of live rock. Then you have to let the tank go fallow for quite some time. Best avoided by strict QT on all new arrivals! (Especially if you get chromis which are prone to uronema infections.)
5) the fish don't tend to get along as well. Unless you have a huge tank, forget schools of community fish. Marine fish tend to be territorial. Firefish, multiple clowns, and royal grammas can all fight amongst themselves. (as well as many other types)
6) for a reef, you just can't have the bioload you would in a FW planted tank. Instead of plants that eat nitrates, you have corals that don't like them. So think less fish overall than you would for a FW tank.
I'm sure there are many more...but there are also tons of great things about reefing that make it all worthwhile. So enjoy!