Here’s how Steve Jobs explains “Think Different” as quoted in Walter Isaacson’s biography (thanks to Mallesh Pai for the pointer.)
We discussed whether it was correct before we ran it. It’s grammatical, if you think about what we’re trying to say. It’s not think the same, it’s think different. Think a little different, think a lot different, think different. “Think differently” wouldn’t hit the same meaning for me.
I may have been taken in by the GDF but after thinking about this for a day or so I am convinced that I understand what he means, even if he didn’t explain it very well. Constructions like “think X” are used all the time where X is a noun and what the writer really means is “think about X” or “consider X” and especially “join the X movement.” (Think “Think Green”, a familiar slogan that is saying “be enviornmentally conscious.” )
“Eat Local” has a different interpretation than “Eat Locally” which would not make sense in its stead. For that matter, “Think Locally, Act Globally” suffers from excessive adherence to grammatical rules.
What “Think Different” was supposed to convey is essentially “be a member of Team Different.” But I am sure that was lost on most people and has nothing to do with why it was a successful campaign.