The great set piece of Venice — the only place in the city that Napoleon called a salon. The piazza houses the Basilica di San Marco on the eastern end, the Campanile (98 metres, with excellent views) on the south side, the Torre dell'Orologio on the north, and the Doge's Palace leading off to the Piazzetta at the southeast corner. The Procuratie Vecchie and Procuratie Nuove line the north and south sides respectively, their continuous arcades giving the square its distinctive proportions.
Come before 9am if you want the experience without the crowds — the light on the basilica facade and the reflections in any remaining puddles of acqua alta are remarkable in the early morning. After 10am in summer the square fills completely. The cafes under the arcades — Florian, Quadri — charge extraordinary prices for coffee but the orchestras they run are part of the theatre. Sit once if you're curious; otherwise walk through and save your money for the Doge's Palace.