I often talk about critiquing stories and I forget that not everybody has the full understanding of what I mean by this. Critiquing stories is not as simple as saying you like or dislike something. For me, it is about trying to help the story creator reach the full potential for that story.
You have to understand that when someone hands over their story to you, they are handing over a piece of writing that they have spent hours on. Indeed some writers liken their manuscript to a baby; they have spent hours nourishing it with attention; often into the late hours of the night or early hours in the morning. It’s the first thing they think of when they wake up, the last when they go to sleep, sometimes the characters even infiltrate their dreams. For this reason you should try to be as diplomatic as possible, nobody likes to be told bad things about their baby.
If you don’t like something you have the right to say so but follow it up with a suggestion about how the writer could fix the issue. If it’s the entire story you don’t like and this is because the genre is not something you like to read, tell the writer that this genre is not to your liking. The story may not be to your taste, but for other people who like that particular genre it may be just right. This gives the writer the option of finding others who like that genre to critique their work.
It’s also a good idea to say which bits you enjoyed and why you enjoyed them. This helps the writer know which bits are working and whether or not to keep them in their final draft. Try to find some good things to say about the story, as only hearing the bad points may discourage the writer and it can damage their confidence in writing.