[nwomack]

1 -- It's perfect for casual play. I have a long commute that involves a subway, a high speed train, and a taxi. I can play it for 1 minute, 5 minutes, 10 minutes. As a counterexample, another favorite game of mine is 'Tilt To Live', but I can't just play it any time, anywhere, because I need a 10 minute block of time before I start the level (technically I could pause it mid game and come back later, but realistically I unpause and die). But with Angry Birds, I can.

2 -- The 3-star system. You can beat the level trivially, but depending on the level it is somewhere between moderately challenging to kick me in the pants hard to get 3 stars on a level.

3 -- It's funny. the birds, the pigs, the sounds they make, the way stuff crashes. I still snicker after countless hours played.

tl;dr - it's a near perfect combination of casual-play, challenge, and hilarity