Warm and friendly tone - allows building customer relations.
Public reply - you can turn situation from "lose" to "win" by publicly replying to disappointed customers. Others will see that you stand by your product.
Reply time - it is normal for a business to take up to 12 hrs to answer, but the world of social media demands faster reaction. One unanswered angry comment can be seen by several potential leads. Or it can turn into several angry comments if someone decides to join the fray.
Quick reply time.
Live conversation - grants better understanding of the problem.
Simple to use.
Language - your call centre must gather native speakers.
High price - and native speakers cost good money.
Communication skills - in real-time conversations your support heroes have no chance to polish their answers.
Automation - your support reps also do not have time to pull up customer's order and conversation history.