AK Brickster pointed out the most common reasons why the majority on this site did not like them. I think 2 other big reasons were that 1, there was a story included among the theme that some felt told us how to play with our Lego and more importantly 2, this was the first theme after the biggest time gap in Castle history (3 years). The anticipation was so great that when the sets came, the disappointment was magnified. Also as Count Blacktron mentioned, this was when the color swap from classic gray to the new blueish gray was introduced. There seemed so many things wrong with this theme, that it just all added up to a huge failure in the eyes of most here. There has been some things that were well liked by some that have been pointed out. My favorites in particular include the newer faces (with pupil eyes), the Shadow Knight helmets, and the inclusion of the armor known widely as "Euro armor".
Chief Smithy of Brick Forge
I buy LEGO, I play with LEGO, therefore LEGO are not for children, they are for me!