Unfortunately, I can't give any hard proof to back up this answer:
By creating challenges, rather than merely impassable wards, the teacher's traps served to:
- Keep students from investigating
- Weed out lesser wizards that may have heard about the Stone
- Slow down better, more determined wizards, in order to give Dumbledore time to act
In particular, Dumbledore suspected that only Voldemort would be needful enough of the Stone to venture into Hogwarts and pass the challenges. The challenges were made to be riddles/things to be overcome, because Voldemort's ego would force him to complete the challenges, as opposed to blowing them up/bypassing them with force. Since Voldemort "knew" the challenges had answers or solutions, his belief that he was smart enough to solve them would keep him occupied.
Any other methods, such as protective shields, would only have worked as long it took for Quirrell/Voldemort to figure out how to bring them down. Then, once that bit of magic was discovered, Quirrell would bring the shields down and immediately have the Stone.
I do not believe it was Dumbledore's intent that Harry, Ron, and Hermione enter into the trials to retrieve the Stone, as the currently selected answer supposes. Dumbledore was not so irresponsible to have First Years face off against someone he suspected to be working as an agent of Voldemort, or a form of Voldemort himself.
The enchantment on the Mirror of Erised was not targeted to Harry or his friends. It was for anyone who wanted the Stone, but did not want it for themselves. Dumbledore himself would have qualified for this. So, the final Mirror protection was more to key the Stone to Dumbledore or one of Dumbledore's trusted agents, and not so that children could access it.
I believe my theory Dumbledore's plan for the challenges to slow down the thief is implicitly proven with the fact that Dumbledore did indeed return to Hogwarts (from London) in time to prevent Quirrell/Voldemort from getting the Stone, even though Harry had been rendered unconscious.