What's been described here was, and is, the joke. The Nostalgia Critic is a character who is so deeply nostalgic that when he revisits a work from the past and sees that it doesn't live up to his rosy vision of how things were he's infuriated. He is a skilled film critic with legitimate points to make, but who nevertheless is also incredibly immature and easily angered. That's the core premise of the character. The Nostalgia Critic character has since been expanded, and is now more of a caustic critic who will draw any work which is either deeply nostalgic or current (but poorly made) into his sights.