Definitely read Melosu's link for an exhaustive list :P
#1...a lot of 1.023-1.025 salinity saltwater. And a heater obviously. There are people who have several hundred gallon tanks with no filters or skimmers. Just a bunch of macroalgae, live rock/live sand and water changes.
#2 A sump to house all of your equipment, return pumps, etc. I would have a part of your sump dedicated to growing Chaeto, which helps keep down phosphates and keep algae out of your display.
#2 also: live rock/live sand. Biological filtration is key to stability, and with a lot of sand/rock there are lots of places for bacteria to colonize.
#3 filtration/aeration. Actually this is probably tied with #1. Surface agitation, be it from a return pump or powerhead is very vital to oxygenating the water and keeping livestock able to breathe.
#4 activated carbon to help reduce water changes and make water clearer.
#5 powerheads to prevent dead spots in the water.
#6/#2 protein skimmer. Helpful when perfect water quality is required, though goniopora, certain sps
benefit from "dirty" water. In smaller tanks not really necessary as a several gallon water change weekly/biweekly is plenty. Very important in a larger tank to keep water clearer/nitrates down though.
Forgot to mention lights, which are way up there if you have corals or just want to be able to see your tank. Depending on what you have, you may need compact fluorescents, t5s or HQI. Aim for 2-5 watts per gallon in the 10k + range. You could probably go for some 250 watt T5s in a tank that size