Some will be valid criticisms, others are purely subjective. What you do need to watch for, however, are patterns. If five out of seven agents mention that they did not understand your main character's motivation, or that your opening chapter did not engage their interest, then you need to take a hard look at what you're sending out. I began by saying that there is no secret trick to getting published. You can, however, think of the process as a game.
Games have steps, rules, and strategy.
The better you become at these, the better you can use them to your advantage. The good query letter is one of your most valuable assets. Inc. 693 visitors have been here since the counter was last reset.