By what criteria do they judge the Best Screenplay at film awards?

The reason the Lost in Translation script appears weak on the page is precisely because it is a well crafted script. Let me explain. A script is not a literary work in itself like a play manuscript, but merely a blueprint for a filmmaker to use as a guideline for shooting (as are storyboards, which are created after the script is written). A good scriptwriter needs to be able to think in the same way as a director, that is, to imagine how the director might be able to interpet the script using all the audio- visual elements at his or her disposal (relating to mise-en-scene, sound and music, editing, casting and performance). Since the essence of so much good filmmaking hangs on the principle of "show, don't tell", the scriptwriter needs to understand that the script is merely a rough skeleton in which the narrative structure is laid out. He or she needs to understand precisely what NOT to state, or at least to leave the director plenty of room for audio-visual interpretation. In the case of Lost in Translation, the writer and director are the same person, Sofia Coppola, and therefore the writer had a perfect understanding of the way ideas and feelings either literally indicated or implied in the script could be expressed using the tools of direction. That is why the script may not be a riveting read. It's not meant to be.