The Moonlight (and ignore the spurious name, which definitely wasn't given by Beethoven. A piece by any other name would sound as sweet, anyway) is popular for a reason. Many pieces are easy to play, but are not as universally loved. The strength of the first movement is that it communicates an awful lot for the amount of effort one has to put into it. Like Einstein's realtivity theories, the beauty lies in its simplicity - it's ability to say so much with so little. Besides, there's a difference between playing it and writing it.
The second movement doesn't really do much for me. Second movements rarely do. I like the 2nds in the Pastorale and the Pathetique, but mostly I find them dull, and would rather skip right to the payoff. I know this is blasphemy, but so be it. The 3rd movement of the Moonlight I consider to be one of his greatest Sonata moments. Up there with the 3rds of the Tempest, Appassionata, Waldstein, and the first Sonata, which I love. It's also surprisingly difficult to play. I was learing this while learing Chopin's 1st Ballade, and found the Ballade to be considerably easier. Maybe it's because there's more room for interpretation in the Ballade, whereas the Moonlight requires you to race through ten pages, not including repeats, at the same tempo with nary a rest. But when played right, I don't see how anyone can say it isn't Beethoven at his most passionate.