It's just generally well written, very good and solid characters, good en unexpected twists, a very solid (and very original) setting and a very good plot throughout the series. Admittedly though, the plot is at times rediculously complicated and hard to follow, some of the books hardly seem connected to one another and things will only fall into place one or several books later, I suppose that might make it less interesting for many people.
If you don't mind a complicated series, which you'll only really understand after reading it at least twice, with so many characters in it that it makes your head spin at times, it's well worth checking out
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http://www.stevenerikson.com/)