I'd assume, simple would be being able to explain your system to someone who doesn't know much, if anything about finance, and have them understand it.
I believe I could explain to my sister and her husband my system within a few minutes and they would understand, as I can properly define it and everything, although a lot of discretion and intuition is involved.
I think that is a fair definition, what do you think?
I personally believe, after all the knowledge I have... if you can't explain your system to someone who knows very little about finance, and have them understand it even a little, then your system is either automated with very complex algorithms and all that, or it's just fairly complex and involves a lot things....
Of course a complex system isn't that bad if a trader can execute it, but a simple system can be more flexible and I think is better for newbies, as I started with all that complex crap and indicators and it didn't do a thing for me.
We all know this stuff though, blah.