It has the elements, feels good and is missing?
Lets start with the levels. The levels are great. There are over 200 levels which offers variety and you can repeat levels if enjoyed it that much which is a great feature. However, the difficulty going from level to level vary greatly within their category. One level may require a great deal of thinking, planning and pin-point angle adjustments and the next is as easy as launching the bird and it kills everything instantly. A little more attention to the placement of the levels within the category according to difficulty would make the score go higher.
On a positive note, the variety of characters for use is just right. With the use of 5 - 7 birds, all having a special talent involving exploding or a tri-split allow for diversity in planing. This aspect was well portrayed. It offers advantages in certain levels where it also offers a draw back in the next. The choice in their ability is well thought out and planned.
The story is very basic and comprehensible. Although a rather irrational choice on the protagonist and antagonist, it offers that comic relief which is needed when one becomes angry with a level that took them an hour to beat.
Overall the game got a good score however it is far from being perfect. An 8.5 was awarded for complexity within simplicity in design and concept of story, character and game play.