I think there are a lot of factors.
-I think in general, it is hard for a second version of anything to surpass the first version that you experienced, which set the template for what the material "should" be like. If you read the book first then saw the movie, you often like the book more. If you saw the movie first, it's often the reverse. This is especially true if the first version was a really acclaimed performance or an indelible production.
-People who go to the theater a lot and are passionate about it know what the theater is capable of, and want the show to be an "A+" every time. They want the "B" show to be an "A+" (and feel they know how to make it an "A+") while the casual theatergoer may be just fine with the "B."
-People who rarely go to the theater are seeing the show because they really want to see it, so they are pre-disposed towards liking it (without having the almost unreachable expectations that many of us can develop). Many people on the board see a lot of theater, without necessarily being drawn to a particular production that strongly.
-If you go all the time, the "specialness" of the experience wears off, and that "specialness" elevates the enjoyment of the experience. If you had your favorite food every day, it might not seem as exciting to you.
-Similarly, a lot of stagecraft is old hat to us but are exciting to casual theatergoers. (Not just special effects, but fancy costumes, big dance numbers, etc)
-Some (not all, but some) casual theatergoers are more starstruck than hardcore theatergoers who have seen many celebrities on stage, and some are excited just to see a famous person up close.
-Comments posted on the board tend to be extreme in either direction -- if you think something is pretty good but not spectacular, you may not bother to post about it.
Just some thoughts.
"What was the name of that cheese that I like?"
"you can't run away forever...but there's nothing wrong with getting a good head start"
"well I hope and I pray, that maybe someday, you'll walk in the room with my heart"