A friend of mine overcomplicated Notepad. He's very smart and has a lot of things on the go at the same time, so he had a to-do directory-of-directories of text files! Worked for him. I bought him the GTD book and now he uses mail primarily. Works for him and it's a lot simpler.
One big GTD-app problem is that there is enough fuzziness in how you should implement it. This is great because you can just follow the principles in your own way, but it also allows so many interpretations.
Problem #2 is that people are different. So your "simple" is not my "simple". Then a developer follows the workflow exactly and they get comments about "too rigid". I saw one reviewer say a product was not "GTD" because it had search and GTD people obviously don't need search. Another refuses to use an app without search. That's okay, it just means they'll use different applications.