What is wrong with the glossy paper's lines and colors?
It really boils down to a matter of personal taste, so there is no definitive answer, but in general, some readers prefer the way that the artwork looks when printed on matte paper, rather than glossy, because it influences the brightness of the colour work because of how the ink is absorbed into the paper (and to a certain extent how the paper feels to the touch).
There is a feeling that the colours (certainly at the time this thread started) were too saturated, too vibrant, compared to earlier versions of the books - like a badly adjusted picture on a TV screen.
To be honest, I think more care and attention has been taken recently (well in the ten years since this thread started!) to improve the look of the art, and to have it better adjusted to the paper and inks in use.
This isn't limited to the actual original books either; there was a period when images used in other books (reference titles, etc.) seemed to be being recoloured in very flat, bright colour, and it wasn't always very successful.
But recently we have been treated to some spectacular success, such as the Tintin: Hergé's Masterpiece
book, and the Tintin: The Art of Hergé
volume, both of which have been beautifully produced to make the art appear at its best, by combining the black-and-white line art with high resolution scans of original colouring pages from the archives, which really gives a sense of what Hergé himself would have seen and approved of in preparing them.