Plot cannot readily be critiqued without reading a significant portion of a story. While some things may be subjective in someone's preference towards plot types (romance, fantasy, horror, etc.), the elements of a good plot are universal.
The plot of a story consists of five major parts: exposition, rising action, climax, falling action, and dénouement/resolution/revelation/catastrophe. There are different models for short stories, novels, and long epics, but they still follow the same general structure:
Exposition: Here, the audience learns the setting (Time/Place), characters are developed, and a conflict is introduced.
Rising Action: The action leads the audience to the climax. It is common for complications to arise, or for the protagonist to encounter obstacles.
Climax: This is the turning point of the story. The climax is characterized by the highest amount of suspense and tension.
Falling Action: The opposite of Rising Action, in the Falling Action the story is coming to an end, and any unknown details or plot twists are revealed and wrapped up.
Dénouement/Resolution/Revelation/Catastrophe: This is the final outcome of the story. Here the author's tone about his or her subject matter is revealed, and sometimes a moral or lesson is learned.
Different models usually suit the length of a story more than any other factor. Short stories tend to compress elements, or even combine them (i.e. combine the Exposition and Rising Action, and/or the Falling Action and Resolution). Longer epics may rinse and repeat a few of the elements to build upon the climax. But good stories contain all five elements.
Standard 5 Act structure:
Alternate structure showing a shifting rise in tension to the Climax
Alternate structure, the "W" Plot.
Elements to keep in mind when providing feedback:
-Does the story have all of the parts of a good plot?
-Is the exposition intriguing, drawing the reader into the plot?
-Does the Rising Action connect to the Exposition and build to the Climax?
-Is the Climax anti-climactic?
-Does the Falling Action ease the tension that was built up to the Climax, or does it continue to feel like there is another climax coming?
-Does the Resolution tie off any open plot threads?
-Even if there is a sequel, does the Resolution come to a close that allows the reader to put the story down satisfied?