First of all, a well balanced substrate is the most important factor to a healthy planted tank. those substrates that they sell in the lfs are expensive for a reason. but if you feel adventurous you can always do dirt/soil. Second, the amount of light for the plants that you're planting in the tank. Without proper nutrients, you can do everything else right and yet still not have the bloom in the tank you're chasing after. Different plants require different amount of lighting. So go through the list of plants you would like in the tank and look up the required lighting for them. Third, CO2 injection. All plants need carbon dioxide for beautiful growth although some would be fine just without it. Water Flow, some plants, such as the Madagascar lace plant, need certain amount of water flow to deter the growth of algae on the leaves while bringing trace elements that they need. Very broad question and you can go on days and days talking about each specific factor.
