*de-lurks*
I personally don't really see Cullen and Cassandra working as a couple, and I'm not saying that because I prefer him to be with my PC. (My "canon" Warden went with Alistair and my "canon" Quizzy went with Solas.) It's true they are similar in that they are disciplined, dutiful, devout holy warriors who serve the principles on which the Chantry was founded rather than the institution itself... but that's the problem. They are too similar. One of my favorite quotes from Boy Meets World, "Yes, we are exactly alike, and that's the problem. I'm with myself all the time. I don't want to date me."
Now, I know they're not exactly the same. Cassandra and Cullen differ in other ways too. She's very brash, blunt, harsh, and direct. Cullen is much more mellow and mild. Ideally I would say their differences would complement each other, but I actually think these would clash romantically.
Romance-wise, Cassandra is all about the ideal. She wants an ideal storybook romance filled with passion, poetry, grand gestures, and getting swept off her feet. While Cullen is strong, solid, protective, and can act as a knight in shining armor too, emotionally he's much more fragile and vulnerable. Bless his heart, when he likes a girl, he becomes much more shy and insecure. He stuttered horribly around the Warden, and becomes more embarrassed and awkward around the Quizzy. While he also makes gestures to show he cares (takes her to a beautiful lake, tells her of his childhood, and gives her a lucky pendant he's had forever), it's not quite the bold gestures Cassandra needs, and one gets the feeling he needs the girl to respond gently to keep him from feeling hurt or embarrassed.
In his own way, Cullen needs to be nurtured and helped along in romance. Meanwhile, Cassandra also needs to be nurtured and helped along in her own way, but she acts harshly to hide her vulnerability. So, I feel Cullen would be hurt or scared off if Cassandra rebuked his initial advances as she does to the Quizzy and Iron Bull, and Cassandra would be hurt or turned off that Cullen didn't try harder to move passed her tough exterior to get to her gentle side the way a male Quizzy potentially can.
Bottom line: I feel Cassandra needs a man who is more bold romantically, and Cullen needs someone who is more gentle emotionally. While I can see Cassandra and Cullen being good friends and allies, I just don't feel the other is what they're looking for romantically. They're too similar in terms of non-romantic personality (being disciplined holy warriors), and too different in terms of emotional needs and romantic tastes.
That all.
*goes back to lurking*