Tangibles:
Music. The best music I've found for gaming is the soundtrack for WarCraft 3. Why? Because the music was specifically designed to play in the background of a game. We through on the Conan soundtrack for the start of fights and Chant or Rennaissance-music CDs for towns and temples. Key is, the music musn't overshadow the game.
Miniatures. And a battlemat. Mins make combat go so much faster, and make sure everyone shares the same conception of physical relations in the game: who's behind who, who can see what, etc.
Props. Saying this: "you find the journal of a wizard. Reading it, you learn that he summoned a demon, and this demon eventually corrupted and slew him." is ho-hum. But giving the players a hand-written journal and making them read it is cool. Once, when my players encountered a wizard's lab, I took them into the garage--where I'd set up a wizard's lab, complete with candles, spell books, components, clues, and hidden treasure (including a dagger inside a hollowed-out old book, with a blade smeared with glow-in-the-dark paint). Good props make a game much more fun.
Intangibles:
Good attitude. It helps if everyone is there to have a good time. It hurts if everyone is there to level their characters. Attiture (or, more accurately, bad attitude) is why so many RPGA/convention games suck.
Laughter. This goes along with attitude. Games are supposed to be fun. Even mature-themed, dark games are better enjoyed when there's an occassional laugh.
Knowledge. It helps tremendously if everyone knows the rules, understands AoOs and the various Actions, and has memorized their own character's AC and Attack bonuses.
-z