Mostly, I dislike it because it is just the type of speech pattern that bugs me the most: saying something that is almost entirely the opposite of what you really mean but expecting everything to follow along.
Someone says they don't drink. Another person says they drink too much. The armchair philosopher makes a gesture and utters, "everything in moderation" in a slow voice and walks off - certain they have solved the issue.
But it doesn't make sense. Do you really mean that? Should I huff gasoline in moderation? Or drive into mailboxes once a week? Should I pray in moderation? Love my wife in moderation?
No, what the person means is "drink a little alcohol if you like but don't get drunk." The phrase is almost exclusively used for the discussion of alcohol and nothing else, yet people say everything in moderation and expect me to understand that the word everything only refers to one thing. (The most foolish, when confronted, say something brilliant, like "isn't that in the Bible somewhere?" Even Yahoo can answer that.)
(Actually, someone should have mentioned moderation to him when he put on those hammer pants.)
Furthermore, the person who says this while fancying themselves to be a philosopher is apparently not accustomed to thinking things through. Obviously, a person cannot do everything in moderation because that would lead to excessive moderation - thus the premise defeats itself, proving that colloquial sentence fragments do not construct solid world views.
But, whenever I mention this to people they always get quiet and leave. I don't know why. Maybe they are going to tell someone in the next room that "cleanliness is next to Godliness".