These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The green bird, to the point where its awkwardness is lampshaded in a cutscene from Seasons.
Purchasing "The Mighty Eagle" and attempting to get "Total Desctruction" on every level of Seasons becomes problematic when you reach the Chinese New Year levels and the Eagle is replaced with a Dragon, which has a completely different pattern of movement.
The marmosets' ability to regain their balance in Angry Birds Rio, sometimes even after you've blown them sky high.
Serial Numbers Filed Off: The developers reportedly ganked the physics engine used in Castle Clout and Crush the Castle.
Sidetracked by the Golden Egg it's easy to get so caught up trying to get the bonus level by hitting the Golden Egg that the actual level has to wait.
Surprise Difficulty: Cute, colourful graphics, a silly premise, and some maddeningly hard levels.
That One Level: Just how many times have you wanted to curse the level designers while attempting to get three stars on any, just ANY level that goes into this category? The number of such levels is pretty huge, in fact, and it's a lot easier to lose count than rather list them all.
Particular example. Level 3-5 stands out as being one of the most Googled levels, and that should really say something. Heck, a lot of World 3 counts.
In terms of collecting stars, almost all of the levels in "Mine and Dine" (when in some levels, a score of 100,000 still gives you 1-2 stars) and some levels, such as 2-12 and 3-1, are incredibly frustrating.
There's a particularly ludicrous "Mine and Dine" screen where the only way to get a three-star score is to launch a bird backwards off the screen and activate its special action blind.
Golden egg 5... You can go for hours ending up with one pig left every time.
They Changed It, Now It Sucks: A very minor example, pertaining to the win screen. When you beat a level in the 1.0 version, a happy, chipper tune would play. Later updates would change this to a generic win screen with no music.