As Richard just wrote, it's quite possible to complicate even the simplest bit of software (or paper) which is a human issue. A number of years ago, an adviser to my company (who's become a trusted sounding board since) told me that he believed there were two kinds of people in the world: simplifiers and complexifiers. I've come to believe that and also feel that part of my GTD journey has been to find balance along that axis (I'm a born complexifier).
That said, there are, in fact, complex applications that I've evauated and run away from because they take something that is, at its essence, simple and make it much more complex than it needs to be. FWIW, my current choice is iGTD on the Mac because it is a simple app for managing my projects and NAs. It has a lot of extra bells and whistles for automation that I suppose are very nice if you're into that but they do not intrude on what is an elegantly simple UI that works very nicely.
I've also been using a set of Levenger notebooks to organize my reference materials and as capture devices for my various consulting and writing projects. The ease with which I can modify their contents won me over as did their ability to be folded completely over (unlike a hard bound journal or binder).
I'm pretty happy with the simplicity level I've achieved but continue to look for ways to refine the balance between maintaining the system and working it.