There were several side effects:
1. The monks made The Key into Dawn to make Buffy protect it, resulting in the creation of the worst character in the Buffy universe.
2. Because Dawn was created, Glory was forced out into the open to cause havoc.
3. Becasue of Glory coming, she sucked people's brains out and turned them insane.
4. Becasue people's brains got sucked out and turned insane, the Crueller was summoned and killed some of them.
5. Because of the insane people being killed, The Knights of Byzantium were alerted that Glory and The Key were out there, so they came to destroy Dawn to destroy Glory and end the insanity.
6. Because The Knights of Byzantium tried to kill Dawn to destroy Glory, Buffy was 'forced' to give up her life to save Dawn's.
7 . And finally, because Buffy died to save Dawn from the Knights and Glory, the town of Sunnydale was left to the mercy of the demonic forces because there was no Slayer.
If those aren't side effects, I don't know what else can be.
And by the way, why did the monks have to protect The Key for? Why didn't they just destroy it? The Key only existed to open the gates to other dimensions, right? Well, if the dimensions opening meant the world would be thrown into chaos and the gates aren't meant to be opened, then wouldn't destroying The Key ensure that the gates were never opened? Wouldn't that have averted all the side effects mentioned above? If the monks never made The Key into Dawn and simply destroyed it, none of that would have happened.
Simple logic.