Surprisingly good gameplay and replayability.
The basic gameplay requires that you select various Plant species in order to mount a defense against the undead horde approaching your house. You gather sun power (which falls from the sun, sunflowers and some mushrooms) which serves as your resource for planting more Plants. You successfully complete a level by clearing all attacking Zombie waves.
In addition to the normal game mode, are special events withing the main scenario which do not require resource gathering. These add some variety to the gameplay and serve to break up the repetitiveness. There are also several puzzle modes and mini-games which are accessible through the main menu.
Small additions like the Zen Garden (where you can collect, water and grow plants for coin rewards) and the Almanac (where you can view your database of Plants and Zombies) increase replayability- the desire to complete your collection.
The graphics are 2D and cartoony which may not appeal to all. The colors are excellent and the animation is very smooth. There are lots of Plant and Zombie sprites, though the mission maps are very limited (basically areas of your house like the yard and the roof). There is even a Michael Jackson thriller tribute zombie which is awesome.
The sound is good but by no means an orchestra. You have the zombies who occasionally demand "brains" and the catchy music. Personally the real highlight with regard to the sound design has to be the theme song "Zombies on your yard"- a music video played in the credits.
Single Player only but the low system requirements for the PC means you can play a few quick rounds on the go, even at work ;).
So overall this game was actually surprisingly good. I never expected such a simple looking game to be this fun.