Virtually every scene in your story should include conflict. That is, a character should want something and should meet resistance. Ideally, the purpose of the scene should be revealed near the beginning, the obstacles should occur as the seeing develops, and it should be clear at the end of the scene if the character has achieved her goal, failed to achieve her goal, or created a change in circumstances (usually for the worse).
A scene may not have all these elements in the first draft, but they are good to check for when rewriting or critiquing. It is also good to look to see that the resistance and/or stakes increase with each obstacle. (Of course, with each scene, resistance and/or stakes should be increasing for the story as a whole.) These obstacles cannot remain the same, which is why repetitive arguments or simple contradictions don't work as obstacles in a scene.
What goes for rewriting, goes for critiquing. I make it a practice to look over scenes for conflict even when it's not my own work. Often the key to improving the story is finding those places where conflict is missing and the story lags. Sometimes this can be subtle, and I got a surprise recently when looking over someone else's work.
The scene was between the protagonist and her mentor. There is a lot of humor in the scene and warm feelings between the two characters. I got totally lost and it and enjoyed every moment, but I couldn't understand why. The protagonist didn't seem to want anything, and the whole discussion was, on the surface, very agreeable. I had to look more closely to see that what I was reading was more than just a pleasant exchange. In fact, it was a battle. The mentor urgently wanted the protagonist to agree to doing something, and the protagonist was fighting back with jokes, distractions, and compliments. She absolutely did not want to do what her mentor advised, and she was not going to commit to it. So, beneath the friendly surface, she was working hard to change the subject or escape.
All I knew as I read the scene the first few times was that I liked it. I had missed how sneaky the author was, and I almost wrote a note saying that, though I enjoyed the scene, it might benefit from adding conflict. So now I'm on alert. If I really like a scene, and I don't see the conflict, it means I haven't looked at it closely enough. I've been charmed by a clever writer who has something to teach me.