Always remember this - no matter the quality of a movie, there were scores of people (producers, editors, gaffers, interns, sound and lighting, costuming, hair, makeup, etc.) who worked into the wee hours of the morning for months to make it happen.
The job of the critic is to steer audiences in the right direction. So, if something didn't work, give concrete examples. What was lacking in Mr. Johnson's performance? Where could Ms. Sedgwick have improved? What was a particularly poor directing decision? All of these are rife for discussion...what ISN'T helpful is generalizing about bad acting or "predictability".
Also, try to find something positive to say. The people I mentioned above? Most of them don't deserve to be dragged through the mud. There's always a silver lining, and you're not proving to anybody that you're smarter than them by staring down your nose at a project and claiming that you're above it all.