Why do critics unanimously pan this movie?

Rating: 6/10 (22 June 2010)

Critics have consistently called this a bad movie - the rating on Rotten Tomatoes is 25%, the director Ron Howard was nominated a Razzie for worst director, and the movie has been called one of the worst of 2006!
I wouldn't call it bad - but it isn't exactly great. I'd say that, it is at least above average. I would not watch this movie again, but I didn't mind watching it the first time, and I didn't hate what I saw.
One of the problems with the movie is that it's too long - about 2.5 hours - and that, during that time, there are long periods where nothing happens. In that sense, the movie can be boring to the point that you turn to daydreaming, or stop paying attention. The movie seems to piddle along.
Another problem that I have with the movie is that, in some annoying scenes, the characters only speak in French, and you have no clue what is happening.
Also, and I don't know if this is a problem with the movie or with the book, since I haven't read the book, but it seems that the plot to this movie and to Angels and Demons is an almost aimless drifting. Robert Langdon will discover an almost arbitrary thing, and then, through some very random string of arguments on esoteric history and signs, will come to a conclusion that is too absurd to believe, and yet is almost always correct.
Nevertheless, I don't consider this a bad movie. I didn't pay close attention to every detail - I was multi-tasking on the computer - and so I wasn't bored. Honestly, I don't see how you can watch this movie without multi-tasking. It is interesting to see how the plot unfolds, and to learn about history and conspiracy. Things do happen, and while the plot can be a little unreal it still has the consistent flow, with a number of breaks, that you would expect in a mystery. It is not exactly a thriller in the sense that you are at the edge of your seat, although it does have those moments. I would not say that it is exactly a cerebral movie, but it is still interesting to think about.
Overlooking the plot, I think that everything else about the movie was well-done. The musical composition, the camera work, the scenery.