(→Acceptable reversions: follow link)
The main purpose of reversion is to undo vandalism or other [[WP:Disruptive editing|disruptive edits]]. If you see an edit that you're sure was intended by its author to damage Wikipedia, and it does, there is no need for further consideration. Just revert it.
In the case of a good faith edit, a reversion is appropriate when the reverter believes that the edit makes the article clearly worse and there is no element of the edit that is an improvement. This is often true of small edits. Edits that introduce [[wp:undue|undue weight]] should be reverted until consensus is built.
Whenever you believe that the author of an edit was simply misinformed, made a mistake, or did not think an edit through, go ahead and revert. If that editor (or anyone else) re-reverts, you will know it is more than that, and you should be more conservative in deciding whether to revert it again.