Perhaps the writer already has received some interest in their manuscript. It can help to have shorter pieces published first. Agents read a lot of journals, magazines and newspapers keeping an eye out for new and talented writers. If you are sending your manuscript to more than one agent at a time, let the agents know. There are numerous short story and manuscript competitions to enter.
Agents often note award-winners.
An indication that you have researched the consumer market you are writing for can be an advantage. There is no such thing as a 'general' market amongst book-buyers and a work which is written for an appropriate and targeted market is always more appreciated than one which isn't. Be patient but not timid. Most agents will give you a timeframe in which you can expect a response and they do their best to respond within that time but occasionally run late. In this event, give them a little more time, then make a polite phone query.