3D is not offensive to me. It just not something I think is necessary to enjoy a GOOD movie. Maybe it helps adequate or crappy movies to gain some traction with the audience, I don't know. But as you say, you can't polish a turd.
To me what makes a good movie is the building of great characters, a good story that either moves me or teaches me something. Good acting is important, as is good directing, and finally someone that can keep the cheese factor down.
I saw Avatar in 3D in the theater, because I was curious. No visual miracles in the world can save that movie from being a lousy story. Like everything else it's another way to wrap people in a neat package of 'entertainment value'. I hated the movie, and the added visual tricks did nothing to enhance the experience.
Now to the flip side of the coin; if I were to see a really good movie in 3D, I might enjoy it. But I would enjoy it in 2D too without missing the possibility of seeing it in 3D. So for me it's completely unnecessary and frankly a waste of time.