To be fair, “Think Different” could also be parsed to mean “Think about things which are different”. In that case, it’s still a fragment, but that’s to be expected in advertising slogans, and it’s otherwise grammatical.
How is it a fragment? The subject is understood in imperative sentences.
Eh, whenever someone says something like, “adverbs are real nice,” I can’t help but think of depression-era Chicago gangsters. No real reason, I just assign that clipped adverb to them. If not to the gangsters, I assign it to a an overboard Texan… like “right proper of you.”
Part of it may come from not really getting the use of the term “real nice”… I’d usually use “very,” “really,” or “good/well.”
Careful writers shouldn’t use adverbs.
Write with verbs and nouns.
If you MUST use an adjective, OK.
I once wrote and ad for a natural insect repellent which said, “If you knew what’s in most repellents, you’d prefer to get bit.”
Needless to say, school marms kept calling me up and complaining about my grammar. “I know it should be ‘bitten’,” I’d tell them. “But if Shakespeare can write about the “most unkindest cut of all”, surely I can be afforded literary license to sell my product.”
This mollified them.
He’s a Pict, not a Hibernian (i.e. Scottish, not Irish).