Heartless,” which opened on Monday night at the Pershing Square Signature Center, provides only flashes of the glorious theatrical glee and anguish that animate those plays. Directed with lots of air-devouring pauses by Daniel Aukin, with a stark black set designed by Eugene Lee, this Signature Theater production calls ponderous attention to its great metaphysical themes. And its symbols stand out like road signs on those lonely stretches of highway that Mr. Shepard so loves to write about.