Existentialism is grounded in the insight that if A equals B, then A minus B equals nothing.
Existentialism is a French school of philosophy popular for a time during the 1950s and 1960s that taught its adherents how to wear a black turtleneck, dangle a cigarette from the lips and strike just the correct pose of dour worldliness-all useful tools for seducing female graduate students over coffee.
Existentialism is the idea that one is entitled to a side dish of either coleslaw or potato salad, and the choice must be made in terror, with the knowledge that not only is our time on earth limited but most kitchens close at ten.
Existentialism is not so atheistic that it wears itself out showing that God doesn't exist. Rather, it declares that even if God did exist, that would change nothing.
Existentialism is about being where you are and taking responsibility for who you are -- be that physically or imaginatively. An existential map has 'you are here' written all over it.
Existentialism is, (as captured in Sartre's famous phrase: 'Existence precedes essence') wrongheaded in that it is simply a reversal of plato and is thus still a metaphysical statement.
Existentialism is wrongheaded in that it discovers freedom within a prison, a demand for commitment expressing its impotence, and a personality that realizes itself only in suicide.