The ponies on this show have revealed too many major weaknesses in their own character:
Twilight Sparkle thinks too much and takes life way too seriously. She even a bit of a control freak that goes off whenever the slightest thing didn't go according to her plans.
Applejack is so single-minded. She focuses more on the task at hand that she fails to see what goes on around her.
Rainbow Dash is blunt, arrogent, and egotistic. She thinks her fast flying make her rule above all others, and she prefers to act without thinking.
Rarity is obsessed with her own status and image. She believe her creations are more precious than life itself.
Fluttershy is so easily frightened. She only thinks of her limits and believing that there is nothing that she can do.
Pinkie Pie is too focused on her shenanigans to take anything seriously.
Spike is insensitive, narrow-minded, and on occasion; immature, unreliable, and hardly ever listens.
Princess Celestia is not as powerful as they say she is. She can help others, but she can never help herself.
The Cutie Mark Crusaders doesn't know what really important. They want the prize, and not the lesson that comes with it.
If these ponies are not careful, their flaws will be the end on them.
They are all so naive. No wonder they were easily fooled at the wedding.