PvZ is one of those games that budding designers should consider studying. The mechanics are all so intuitive and clear that anyone can pick the game up and quickly understand situations that would otherwise be quite complex, and the game is good at forgiving initial mistakes but also good at punishing really fundamentally bad decisions by the player.
I think the only real complaint is that in the end, for every level type there's a stand-out set of plants that, used correctly, will absolutely destroy any zombie formation that you'll encounter on that level, and once you understand that the game becomes a bit simple. It takes a while to get there though.
(Also the Vita version is secretly the best with its feature to tilt the screen to collect sun, I'm not sure if that made it to any other system - I know it's not in the current Android version).