I treat Zynga as a late "me too" company. Look at it this way:
FarmVille = toned-down SimFarm
CityVille = toned-down SimCity
Mafia Wars = toned-down Grand Theft Auto
To me, they're all just toned-down versions of said originals. As for playing them? Why would I when the original inspirers are still available?
And as for Angry Birds, it's too simple for me. The factor that things like Tetris were developed with mathematical attributes in mind may have something to do with this. I found AB to be "predict the trajectory angle" ala Human Cannonball for the VCS, while Tetris provokes me to think on multiple levels- "Do I fit the z-piece above the t-piece? Will the line piece mess up the balance of the buildup on the left? Will the high-speed mess up this l-piece landing onto the square, or should I interlock it with the other l-piece to score a double-line?"
All depends on time and thought process, IMO.