WOW API - Creating Addons

I have been playing wow for years and using various addons. Also, I am developer and I wanted to dig into wow API and lua programming language. For several days I was researching and studying the API and lua and I am ready to create whatever addon. Knowing the general programming paradigme, I learned the synatx and the API quickly (I must tell the lua is so easy to learn). That said I have to ask what is the Warmane policy about addons? What API calls are forbidden, other then protected ones (that cant be used by normal player)?