It wasn't nearly as crowded at the Acropolis as at the Pantheon, but that is mostly because the space is so much bigger there. I actually visited the Louvre this summer, and (kind of) got to see Mona Lisa. Like you said, there is always a huge crowd of people blocking the painting, but I managed to get a quick photo, just so I could look at it later ;) It was crazy, I lost the people I came with the second we walked in the door, because people were very pushy and you really have to push back to get anywhere in there.