From the silmarillion it's main evil seems to be that it saps individual wills and replaces them with the will of Sauron. Additionally it has a nasty habit of getting it's wielders killed if they don't use it enough to have their wills sapped.

There are a lot of ways to respond to this question, but let me offer two perspectives:

1) The main power of the One Ring is to be able to enslave the wearers of the Nineteen Rings of Power or Great Rings, that Sauron had created during the Second Age of Middle-Earth by the elves of Eregion. During this time, Sauron wore the guise of Annatar, the Lord of Gifts. The Elves created the Nineteen Rings and Sauron distributed them to the lords of Elves, Dwaves and Men. He then made the One Ring, which had power over the wearers of the other rings, hence the opening poem:

One Ring to rule them all and in the darkness bind them.

You can see how well this worked by the Nazgul, who were the leaders of men and continued to wear the rings that they had been given. Sauron bent them to his will and they became his most dire servants.

2) Now stepping outside the world of the books themselves. Tolkien wrote the Lord of the Rings between 1937 and 1949, the years of World War II, when Hitler was trying to take over Europe. Tolkien had fought in WW I himself. That was certainly an influence on the work. Tolkien was also an ardent Roman Catholic and the Ring certainly represents temptation. He said of his work that it was "a fundamentally religious and Catholic work, unconsciously so at first, but consciously in the revision."

Tolkien also drew from non-Christian sources, including the Finnish Kalevala and The Ring of the Nibelung. In many of these stories, sorcerers hide their "heart" or power in an enchanted object for safe keeping. That is why Sauron is destroyed when the Ring is cast into the fires.

So, the Ring is bad because it has power over people (the Ring Bearers), because it holds Sauron's power or heart, because it represents temptation.