* Even if you find an article was slightly better before an edit, in an area where opinions could differ, you should not revert that edit, especially if you are the author of the prior text. The reason for this is that authors and others with past involvement in an article have a natural prejudice in favor of the status quo, so your finding that the article was better before might just be a result of that. Also, Wikipedia likes to encourage editing.
* Do not revert a large edit because much of it is bad and you do not have time to rewrite the whole thing. Instead, find even a little bit of the edit that is not objectionable and undo the rest. (To do this, you can use the "undo" button, then type back in what you want to keep). If a supporter of the reverted edit wants to save more of it, she can re-edit in smaller pieces and the article can converge on a consensus version that way.
==Alternatives to reverting==
The first and foremost alternative to reverting when you find you disagree with an edit is to find a third version of the text that incorporates at least some of the elements of the prior text and the current text. Sometimes that's as easy as making the article state that there is controversy about something.