-Add as much heat as you want by regulating the amount of chile ribs and seeds that go into the paste. If you remove them all, you will get a mild curry, but if you leave some in, the curry will be fiery. If my chiles are hot, I usually only add some chile ribs and seeds.
-If you can find galangal, fresh or frozen, use it! 2 Tablespoons chopped is a good amount. I've left it out of the recipe because it's hard to find and I've made it without it with great results. But if it's available, it makes the the curry even better.
-Some of the ingredients are difficult to find (I know because I used to search high and low for kaffir lime leaves until I just bought a tree so I can pick some leaves whenever I want!). The substitutions won't necessarily be authentic, but they do taste great and will make a delicious curry paste for all your Thai recipes.
-Cilantro roots are just the white roots you sometimes see still attached to bunches of cilantro. If you have access to them (you can often find cilantro with roots attached at farmer's markets), just cut from the top of where the stems arise and rinse the roots off very well before using.