Adam Bosworth has a good post on architecture/design and the KISS principle (thanks to Tim Bray for the pointer). I have to reflect that the most successful software I've been involved with has followed this, and the least successful has not. Coincidence? Possibly. I think there's more to it than that, though...
KISS has lots of deep ramifications. Simple systems are easier to build. There is less to maintain. Less pieces to go wrong. It's also easier for other folks to jump in and play - they don't have to learn a whole new universe to get started, they just go.
Actually, APOLLO is able to work on both sides of the spectrum - some rather complex bits, based on a rather simple overall architecture, providing ways for developers to easily build simple apps...