Two more problems that lead to abandonment:
Gifting: People seeing an animal and thinking that it would be "perfect!" for their friend/family member. People should realize pets should never be given as gifts unless there recipient wants the animal and can care for it.
Pet store irresponsibility: It's not just the buyers being irresponsible. Why don't pet stores have display cards on cages with the requirements and adult size of the animals. How many people have bought a cute little water monitor on impulse, and not one person in the store mentioned that they could have a lizard comparable to a medium sized dog in less than two years. Sure, the buyers bear the responsibility of learning, but it's also the pet stores responsibility to not send an innocent animal to its death.
But we're all just preaching to the choir. Most people reading these posts already have strong feelings about these problems.