Bread made with yeast is an exception though. For most baking projects (cookies, cakes, brownies, etc), there are at least two very important things I know about (and I'm far from an expert):
1. The chemical reaction to make it raise. As I said, yeast is different; but if you're using soda, you need the right amount of all the ingredients (soda, acid, salt, etc), at the right time (water starts the reaction, which will fizzle out after a certain amount of time), at the right consistency (over-mixing will screw things up too).
2. You need to balance wet and dry ingredients. So you can't (for instance) just remove sugar or add flour without also removing or adding something "wet", or the balance gets thrown of and everything changes.
For yeast-based bread you're right -- I normally just dump in water until it seems about the right consistency.