Alas, but Quirrell might have stupifyed the whole chess set, XD

Actually Hermione read ahead, that is why neither Harry or Ron could do the spell. We do not know for sure that the teachers can make whole rooms unplotable, after all the room of requirements was done hundreds of years ago by the four greatest wizards/witches of the age. The room was not locked for the same reason that the Forbidden Forest is not locked, Dumbledore trusts his staff and students to do what is right. To be fair the Philosopher's stone was quite well guarded at least by the Mirror of Erised. All the other things were weak so that one of the teacher's could come and get it out when the moment of danger had passed. When you think about it the only reason Quirrel had half a chance at getting the stone was because Harry went down there.

Ya some of the tasks might of been easy but the mirror of erised was the best thing guarding quirell would of been down there forever if it wasnt for harry coming down and getting Dumbledore is a genius nothing gets past him

I don't know. Maybe thats jk rowlings first mistake.

Good point.

They didn't guard it more because they thought quirrell was on their side and I expect none of them expected voldemort to be in the back of his head.

I was hoping someone would point out that Hermione was reading ahead. Its in the Standard Book of Spells, which is in the required list of books they had to buy for first year.

Quirrell was really good at pretending to be the man he wanted everyone to believe he was. A bloodied leg on Snape's part is kind of suspicious, especially once you know about the three headed dog and the fact that usually trolls are too stupid to find their way into a castle undetected.
As for the test for the Trio thing, Harry wonders about that too. Was it really? I think it was only to the extent that Dumbledore found out what Harry and the others were doing and he trusted them to work together. I don't think that the tasks were made specifically for them. That would be rather silly. Not all wizards are good at Herbology, and flying, and strategy, and fighting trolls, and logic puzzles. Just like how not all Muggles can identify plants, do physical activity, strategy, fighting and logic puzzles. It would take a team effort to get through those things, especially for first years. (Remembering that Quirrell is gifted with trolls and has Voldemort to help him and is in fact an accomplished wizard in his own right.) Dumbledore's mirror is kind of the clincher. Most people would want the Philosopher's/Sorcerer's Stone for themselves. If any part of the challenge was meant for Harry, I think it was that one.

I was just thinking about this today and I think it's because no one even knew about the Philosopher's Stone to begin with (apart from Voldy and the professors of course), and that the obstacles weren't easily overcome. If it wasn't for Harry's quidditch skills, Ron's understanding of Wizard's chess and Hermione's brains they wouldn't of been able to pass the enchantments!