Kottke just turned me on to my new favorite word, Mamihlapinatapai, which is listed in the Guiness Book of World Record as "the most succinct word" and which Wikipedia claims is the among the toughest to translate. In short:
It describes a look shared by two people with each wishing that the other will initiate something that both desire but which neither one wants to start. This could perhaps be translated more succinctly as "eye-contact implying 'after you...'". A more literal approximation is "ending up mutually at a loss as to what to do about each other".
Brilliant.
I can't believe the Germans don't have a word for this.