I think in the past 20 years, we actually have not made as many breakthroughs in game mechanics (of course, this was easier in the early days). Games are still fairly similar to those of the 90s, only technically more complex. I think we are beginning to maybe get a better handle on game design, which I am seeing creep in with more appreciation of more obscure (now less obscure) game types like roguelikes and CCGs. Little known fact is that Plants vs Zombies was largely inspired by deck building mechanics found in Magic the Gathering, so you are beginning to see some more obscure mechanics like that get mainstreamed.