I would even say that part of your criticism of GotM is valid for the entire series. The series is intended as a mosaic of sorts, with a ridiculous amount of characters, races, gods, kinds of magic, etc., and part of the appeal is intended to be figuring it all out - the history, the magic, the intrigues past and present. Your whole "coming late to a D&D campaign" analogy works to a lesser extent for the series as a whole. The history of the world and the various events leading up to the series itself - particularly those involving Kellanved, Dancer, Dassem Ultor and Laseen, but not only those - would not have been so interesting if it had all been told at the beginning, but by letting the reader figure it out piece by piece, a few tidbits in each book, it becomes much more interesting. If you're feeling uncharitable, you might say that if GM Erikson had actually explained everything beforehand to the latecomer to the campaign, the latecomer might've found it a bit boring, whereas having to figure it all out as the campaign is already moving makes it exciting.

But yeah, it's true that GotM was not the best possible introduction to the series, and went perhaps too far in throwing the reader into the deep end of the pool without teaching him to swim (though some people do get through it from the first time and don't get what all this having to try five times business is about). It has often been pointed out that GotM was written long before the other books, and that Erikson's writing skills improved a fair bit during that break.