It goes something like this. . .
During the time before The Lord Ruler, Preservation was weakening. Ruin was gaining power, but it was still trapped in the Well of Ascension. The Lord Ruler then went to the Well of Ascension, took the power for himself, and used it to make Preservation the dominant force in the land. (Among other things, like making the Koloss and such.) This stifled Ruin's small influence and made the Deepness into the mists, until a thousand years later, when Ruin could again reach out upon the world. Then Vin went to the Well of Ascension and gave up the power to Ruin. So the current state of things is Preservation, enhanced by The Lord Ruler's use of the Well, and Ruin, enhanced by Vin's use of the Well, are fighting over the land to establish control.
As for the Well of Ascension, I think it could have been made to make the world a better place. The people, every thousand years, would in theory send the Hero of Ages to the Well of Ascension, where they would then take it's power and use it to improve the world. Sometime in the past, someone just decided to use it's power to seal Ruin away completely. Of course, we've seen that if the wrong sort of person takes possession of the Well, the world can be in for some serious troubles.