The stone was mostly not well protected.
Except for the mirror, the other so called protections are merely obstacles. Three first year students were able to get through these obstacles. So if a few first years get through, Voldemort should have no problem.
In the book Voldemort wasn't able to learn about the Cerberus, but there have to be some books that describe a Cerberus. Maybe Dumbledore removed them from the Hogwarts library, but there are other sources for books. There is the legend of Orpheus and Eurydice, where Orpheus managed to get past the Cerberus, and as many legends in the Harry Potter Universe are considered to be based on wizarding events (Flamel, Merlin, Circe, ...) the wizarding world should know more and not less than this legend.
The only real protection was the mirror.
Men have wasted away before it [the mirror], entranced by what they have seen, or been driven mad...
So Quirrell might have waited before the mirror until Dumbledore comes back, or he might have realized that it is futile and leave, but he would not have been able to get the stone.
It is interesting to note that Harry, in his misguided attempt to save the stone, overcame the only real protection and almost presented the stone to Quirrel/Voldemort.
It is strange that Harry thinks that it is his task to protect the stone, and even stranger that he thinks he can stop an adult wizard. Whether the adult trying to steal the stone is Snape or Quirrell is irrelevant. The stone offers life and gold, and Harry knows that Voldemort wants the stone. So with one year magical education Harry won't be able to keep an adult from the stone by force, and someone working for Voldemort would not be stopped by the thought of hurting or killing Harry, on the contrary it might be considered a bonus. So with Harry's help, Quirrell gets around the protection of the stone.
Another problem with the protection is that the mirror is the only real protection for the stone, but we know that the mirror was inside the castle during Christmas, because Harry found it and saw his parents in the mirror. Therefor at Christmas either the stone was in the mirror without the other obstacles, or the stone was unprotected behind the other obstacles. The books don't answer this question, but either way the protection is worse than it could be.
There are some obvious improvements for the protections described in the book:
- Don't place the key in front of the door locked by that key. Don't place brooms in front of the door to aid in catching the key. Bring the key when you want to go the the stone.
- Don't include a riddle to explain the potions. Do not keep the potion there at all. Bring the potion when you want to go the the stone.
The conclusion is that the protection could have been better.
In reply to @TylerH "being in a locked room in Hogwarts which was not publicized by anyone"
“And finally, I must tell you that this year, the third-floor corridor on the right-hand side is out of bounds to everyone who does not wish to die a very painful death.”
This is not exactly subtle. This wasn't announced in later years, so it probably wasn't a regular occurrence before, so it draws attention.
If Quirrell didn't know where to look before that announcement, now he did. Of course, he already knew because he placed the troll there.
As Fred and George have been in the Forbidden Forest before, they probably checked on the forbidden corridor within the first week, just because they were not supposed to do it.
“First years should note that the forest on the grounds is forbidden to all pupils. And a few of our older students would do well to remember that as well.”
Regarding the other part, first entering Horwarts, Quirrell already managed that part. Something was killing unicorns in the forest, the Centaurs knew that Voldemort was in the castle. So Dumbledore probably also knew, and therefor the "entering Hogwarts" part is no longer a problem for Voldemort.
Note that I didn't claim that the mirror was a perfect protection, just the only real protection compared to the others.
In my opinion a better protection would have been to keep the stone in his office, or in his bedroom in his drawer of socks.