Well... that's a really big question. There is a lot of factors that go into giving a specific answer, but I will give you some general ideas. Mind you that I've only been reefkeeping for a little over a year, so I'm no expert, but I'll tell you what myself and others seem to believe works.
There are (4) main factors:
- Water Quality / Parameters
- Livestock
- Lighting
- Financial Investment
1) Water Quality
With corals, like you said, water quality is certainly important. The tricky part is that depending on what corals you have can change the way you think about your water. In general, "Beginner" corals such as Mushroom, Zoanthids, Palythoas, and other soft corals like "Leather" corals, can live and thrive in "dirty" water. On the other hand as you go up the so-called difficulty scale, it is often said that Corals such as LPS (Large polyp stony) and mainly SPS (Small polyp stony) need much cleaner water with very low phosphates and Nitrates present.
Parameters are also different in that with corals, Calcium, Alkalinity, and Magnesium also become more crucial as corals use these elements to build on. In some cases it is beneficial to switch salt brands i.e. from Instand Ocean to Reef Crystals. They generally claim that it is more beneficial. Also the Salinity will generally be higher. Normal FOWLR is 1.023 whereas a reef is 1.0255 - 1.026.
2) Livestock
It is important to consider this when thinking about growing corals because there are a vast amount of "non-reef safe" inhabitants. Some fish and inverts are knows to eat or hurt corals, or the "clean up crew" (CUC).
3) Lighting
Along with water quality, lighting might be the most important to consider. The same theme holds true however, that typically corals such as softies, mushrooms, etc. only require low lighting conditions (Some growing in dark caves). These corals can live under T8, small 1-2 bulb T5's, Compact flourescents, etc. while most LPS and SPS require very high light conditions provided typically by multi (4-8) bulb T5 units, a high quality LED system, or Metal Halides.
4) Financial Investment
I add this in, because quite honestly, it is a true factor. Creating a reef can cost a great deal of money in the corals alone. Some people offset the initial cost by purchasing frags, or fragments of a large coral, typically 1-4" depending on the type of course. Frags range from $5-50, whereas a full coral can cost $20-$500.
My 54 gallon reef contains several hundred dollars in coral. My small biocube is around the same.
The corals certainly add beauty to a tank, and I'm in no way trying to discourage, but it is nice to know where you stand.
There are many other specifics that really depend on what you are looking to house, but that should give you something to start with. A google search, and even searching the forums can provide you with 1000's of pages and articles that can break down the information much better than me.
Good luck with whatever you decide. I'll still be following along!