Today I began teaching a workshop CONFLICT AND THE CATEGORY ROMANCE. I'm not going to give away the farm of the workshop because people paid good money to hear my thoughts. :) But...it does make me want to say...
All books are about conflict.
When we say conflict referring to a category romance novel, we are primarily talking about about a struggle to fall in love.
So maybe your thriller isn't primarily about a struggle to fall in love, but it is still about a struggle. So is your mystery. Your paranormal. Your steampunk. Your coming of age YA.
People like struggle. Maybe not so much in our real lives, but we respect and appreciate struggle in our fiction. Because we've all struggled and felt the glorious feeling of victory after we've overcome, we understand the emotions involved and we can live vicariously through a character in a book who is struggling. We especially love the victorious feeling when the characters have overcome -- in the happy ending or satisfying conclusion.
Are your people struggling? As I said, in a category romance the main struggle is to fall in love...or avoid it...thinking it's not the right thing. But in your thriller your characters should be struggling. Your protagonist in your mystery should be struggling to find answers. Your protagonist in your coming of age story should be struggling.
And through it all, your protagonist should be growing.
Because no one emerges victorious after a struggle without growing.
Which is really what readers want to see. They want to experience an adventure of struggle, which results in real growth and ultimately a victory.