What so many other posts have said:
1. Surface area. The larger the surface area the more juice is in contact with the heating element and the greater the volume of vapor produced. The downside of surface area is its mass which determines the time energy must be applied to heat the coil. Thus a 32ga round coil will heat faster than a 22ga coil of the same resistance because there is less mass to heat up.
2. Resistance. V^2 / R = W (voltage squared divided by resistance equals watts) and watts are heat energy. The more heat the faster the liquid is vaporized and the warmer the vapor produced is, up to the point of 'frying' it. The downside of high-energy systems is short battery life. I know some really experienced sub-ohm builders who build down to 60 milliohms, but they change batteries as often as I used to change my 510 stick batteries (several times a day).
3. Wicking. Wick characteristics matter. The more rapidly a liquid can travel through by capillary and thermal gradient the more juice is available to be vaporized. Steel mesh is great for high heat, but it doesn't support high juice flow, cotton gives great flow but isn't very tolerant of high heat, silica and Eckowool are middle ground materials and have their own issues (they tend to clog resulting in dry hits).
4. Voltage. Voltage is potential, the higher the potential the faster the current can heat the coil, hence a given coil will heat faster on a fresh battery at 4.2 volts than it will on a "dead" battery at 3.7 volts. Long ago we used to 'stack' batteries to increase the voltage but at the currents we use this was a more dangerous practice and is not recommended anymore.
5. Airflow. Airflow is another determinant of vapor density. Too little airflow and the vapor burns at the surface of the coil, too much airflow and the vapor density drops to the point of sucking air. Getting the balance right yields a vapor that has the characteristics that bring out the flavor/vapor/throat hit of a particular set of juices. Most of us have RDA's drilled out to balance airflow for the builds we put on them, some need big holes 1/8" (or larger), others work best with a couple of #54's.
6. Juice characteristic. Different flavors taste better at different temperatures and densities. The mix ratio of PG/VG also has a lot to do with vaporization temperature of the juice. The components of the flavor also affect the vaporization temperature and there are some components that are not very heat tolerant and will 'caramelize'. As a rule-of-thumb darker juices will caramelize more than clear ones, but as with most ROTs YMMV.
Those of us who build RDA's and do sub-ohm look to optimize these parameters to provide a satisfying vape. Any one of us can talk about builds that fried a particular juice (and not all of them are sub-ohm
), and ones that failed to produce the vapor temperature density and flavor we were expecting.
Sub-ohm vaping is a science and an art. Blowing clouds is a part of it (the fun part), but it's really about finding that balance between vapor density, temperature and taste that makes a juice come alive. That being said, it is not something to jump into without experience. At a minimum you need to be able to understand battery safety, build characteristics (above), and the characteristics of the mod and atomizer (voltage drop, current path, heat transfer) being used. If these things are not in your repertoire of knowledge, keep reading the forums and ask questions.
