Participial phrase as an adverbial: "expressing their dissatisfaction" the "expressing" functions as a participial phrase, used as an adverb to describe the customers' behavior, adding descriptive and complexity to the sentence.
Direct speech structure: "they’ll tell the staff directly about what upset them" uses a direct speech structure to convey the customers' specific words and feelings, increasing the directness and authenticity of the sentence.
Conditional adverbial clause: "either on the spot or through a phone call" provides different scenarios where feedback occurs, enhancing the situational sense and specificity of the sentence.
Coordination structure: "It's not only about venting. Many also make a point to request improvements" uses coordination structure to show the dual purposes of customer feedback, emphasizing that it is not just about expressing emotions but also about expecting improvements.