KISS, otherwise known as Keep It Simple Stupid is something that has been drilled into me at least since High School. Don't over complicate things needlessly. If you can find a simple solution that works, use it. Complicated solutions may impress yourself, but they aren't going to be fun for anyone that comes behind you.
This is not dumbing things down for The Lowest Common Denominator (which I've come to believe is WotC's plan for D&D 5e). Dumbing things down doesn't just change the process, it changes the result. KISS is an attempt to attain the same result using a simpler process.
Most of the OSR is built on the KISS idea. D&D / AD&D may not have been the most clearly written games, but the OSR rewrites have been able to attain the same results with less confusing "packaging".
Savage Worlds, and even more so Hollow Earth Expedition, really exemplify KISS. It's the reason I'm going gaga over the two these days.
Apocalypse World, on the other hand, takes a very simple system and surrounds it with obtuse wording and descriptions. It makes a system that by it's nature is KISS very non-KISS like. Which is a shame. As I've said before, AW plays SO much better than it reads.
Sometimes we are too attached to our own creations, no mater how large or small, to see that we didn't give KISS a chance. It takes a second set of eyes to point out the shorter path to the same destination. This is what happened when Keith Davies took my Critical Hit table, cleaned it up and removed the need for an additional roll. Clean. Quick. Simple.
I'll Take a KISS over additional complications any day of the week ;)