Hmmm. Who defines what is hip?
That question is beyond my power, is a very complex mess of common knowledge, media influence and "opinion-makers" (not evil corporations agenda, just influential people). I had like 4 years of classes on that subject, and the best answer I got was "depends". I.e, why the alien from Space Invaders became the symbol of old games? Because it was more popular? Easier to copy? Midway was less anal about copyrights than Nintendo? A very influential magazine used it and everyone copied? It's really ahrd to say without examining each case, generalizing and making a golden rule only makes it worse. Think of it as the memes on the internet, why some become popular and other not?
IMHO, it has to be iconic, like a symbol, so people will reckonize it and know what you're trying at. Wear those "funny" Space Invaders or Oregon Trail shirts and you're showing off how
douche douche douche douche co-douche cool you are. They may not get the game right, but they'll get it's a "retro thing". If you wore a XCOM Sectoid t-shirt, no one would get what you're talking about, and it doesn't look "retro" enought to be cool; they can't safelly say it's from a old game.
Same for retro games artstyle, they have to be from a style everyone is familiar with... Cthullhu saves the World is obviously like old FF's, and that's a reference that l33t gamerz understand, but they never saw Ultima, Gold Box games or Wizardry. If Cthullhu saves the World were a Wizardry-style game, I doubt it would have even half the success.
I had a marketing teacher that had a wonderfull saying: One thing is a man dressed up as a bat fightning a clown, the other is Batman versus the Joker. I think that sums up everything.
