As a start, here are a few things I have learned:
1. All individual plants are different. Even if it is just a common Christmas cactus, it WILL have its own ideas about what it should do. I've come across people who grow some of the exact same plants I do, and their plants sometimes go dormant, bloom, and grow in wildly different patterns than mine.
2. All growing conditions are different. Even if you are growing the same plant under what appear to be identical conditions to your next-door neighbour, the plants will not be the same. Subtle variances in light, microbial soil make-up, and water can result in vastly different plants. It does not mean that you (or your neighbour) are doing anything wrong.
3. Assumptions are usually wrong. Assuming that 'cactus' need as much bright light as possible can cause problems sooner or later. Most succulents do respond well to bright, indirect light. However, many, if not most, will shrink away, and often burn, from direct, hot light. Similarly, it is dangerous to assume that succulents need as little water as possible. I have lost plants to too much water, but I have also lost them from too little. When in active growth, and especially in hot, dry weather, succulents can require a surprising amount of water to thrive.
4. Cultivation is different than habitat. Whatever a plant may achieve, or be able to tolerate, in habitat will be very different from what it can do in even the best cultivation conditions. A Saguaro in habitat may be able to survive extreme heat and extended periods of drought. However, once you put it in a pot (or garden outside of its normal climate), it's a whole new ball game. In such a situation, a plant will lose quite a bit of its natural ability to adapt to extreme situations, and possibly some of its stature.
