The story is very good, has you at the edge of your seat. The mystery introduced at the beginning is revealed very slowly throughout in an excellent way. The main storyline (the building of the cathedral) is also very good, and you're really rooting for Kingsbridge.
 
Some nitpicks: 
- the novel is very long, and there's some things I feel were superfluous, that didn't further the plot, nor added to the theme 
- the characters are not very nuanced, they're either Good Guys with no flaws or Bad Guys with no redeeming characteristics
- when it comes to the portrayal of women, you can really feel it's been written in 1989 - I felt is was quite male gaze-y despite the strong women characters (who are merchants, self sufficient forrest dwellers, craftsmen, ...)
- there's a liberal usage of the "Good Guys are Beautiful, Bad Guys are Ugly" trope, particularly in the women characters