You're not missing anything, they're conceptually very simple games.
As a result of being so simple however, they are very approachable and accessible even to people who are not traditionally gamers. Many of the greatest games out there follow a rule of "simple to play, difficult to master", and the same is true of Angry Birds -- the different types of birds can take a while to get the hang of, and are sometimes given to you when another would be more ideal for the level in question. A clear feedback and reward system is presented by the points scored, the star system, and bonus items in the levels, and beating some of the more complex levels can be visually rewarding as well, watching everything happen correctly. The star system and golden eggs (which unlock bonus levels and little bonuses such as sound-boards and the like) provide motivation to continue playing, and the high-score system which compares your scores to others can bring out the urge to compete.
Angry Birds also makes very good use of the touch-screens on iOS and Android devices where the game was originally released, and given the relative lack of processing power does not have to compete with traditionally "higher end" games.
I think these games:
- Are not necessarily aimed at traditional gamers, and therefore might do things you have seen before or seem overly simple.
- Are meant to be picked up, played briefly, put down, and returned to later.
- Make good use of the systems they were designed for, but may not stack up against AAA titles for PC or console if directly compared.
Rather than comparing them to traditional big hit games, perhaps try comparing them to games such as Tetris, Windows Solitaire, Zuma and Bejewelled -- all very successful titles, and all smaller, simpler games.
Games also don't have to do anything new to be received well, they just need to be polished and to do what they attempt to do well.
These sort of games certainly aren't for everyone, but I for one enjoy an occasional level of Angry Birds when stuck on public transport or waiting in a queue for something, even though I would probably never consider sitting down for a prolonged play session like I might with a game like Halo or Gears of War.
, Is that it!, try to hit the targets 
, surely theres more? I fear not 
