Just look at the book description. Assassins, competition, tests, fight for freedom, castle, vicious king, magic. It's like a mix of "The Hunger Games" and "Poison Study". So it should be awesome, right? But "Throne of Glass" failed to deliver all that awesomeness. 
 Firstly, all these promises about epic competition between twenty-three best killers, thieves and warriors were never fulfilled. I was expecting battles, struggles, and attempts to survive. But the competition itself seemed more like a background story, secondary plot line. From 23 candidates we were introduced to 4 or 5 maximum. And even at that it was only basic information like name and occupation. 
 The tests itself often weren't even described. We would read lines like "The month has passed during which there were two more tests". But I want to read about them! What they were, how hard it was to pass it, who make it further. It was supposed to be a book about assassins' competition, so give me one! I feel cheated. 
 Secondly, I didn't like main heroine. She was so arrogant and vain that it boarded on unpleasantness. Moreover, I would have forgave her arrogance if it was justified. But she wasn't that smart, or she would have figured the mystery faster, she cheated in one test, she felt for the trap of the duke, and in the end she was saved first by spirits and then by a boy. So why would I consider her the best assassin? Based on her reputation? But not once were we given an explanation how she earned it. 
 And that is another problem. Author didn't give us enough background. Maybe she is keeping it for the next books, but without strong background the story felt shaky. 
 "Throne of Glass" is not a complete failure. It still was somewhat entertaining. It's just that I expected so much more from it.