My first 3 days in Venice, the Square was completely flooded, and I entered the Square from the North East. This did not give me a good perspective of the Cathedral, as I came from a small alley. Plus, it was raining and grey the first few days, so I never really looked up.
My fourth day in Venice, the sky was blue, it was great sunny day, and my companion and I, after touring most of Venice, entered the Square from the West. WOW!! I had no idea how impressive the cathedral really was. It was such a shock when we both came around the corner and the street opened up into the huge square with St. Marks at the far end. We both stopped in our tracks, and couldn't speak for a few minutes.
Visiting here is definitely a 5 star adventure, but you really need to get the right perspective to fully enjoy it.