One of the main differences they have noticed is the GPU is no longer attached to the logic board. The GPU is now present on a separate small card that can be detached from the logic board. This makes part replacement more efficient and cost effective. Which means that if you fry your GPU, you no longer need to replace the entire logic board.
The other difference they found is the optional SSD drive bay, as this is now available for the 21.5inch model. This is located beneath the optical DVD drive. The optional SSD drive was previously only available on the 27inch model.
