This is a short list of things that irk agents most when reading a query--
1. Saying no other titles are comparable to yours because it is too unique.
Well, the thing is your story may be unique to you. All stories are unique in some way or the other, but what this statement really tells me is that you don't know your market, or there is no market for your book. Do the research, find something you can compare to your work.
2. Saying you're the next Dan Brown, Harry Potter.
There are many writers whose books can be compared to Potter or Brown or James Patterson, etc. in some way. Try to find other authors to include in your comparable titles, authors with great track records, but not so much hype.
3. Too much personal info.
You wouldn't go to a job interview and tell that person how much your dog, or your parents, or whomever, loved your ability to brew coffee (or file papers, etc.). So, don't tell me how much everyone loved your story.
On the other hand, if a published author or a known editor has read it and made positive comments, by all means give me a BRIEF run down.
4. Too little essential information.
You can tell me as much about the story as you want, but I need the genre and word count.
In the same vein, you can regale me with your publishing experience, but please include something about the story your pitching me.
5. Addressing your query.
Do not 'Dear Sir or Madam.' Do not "To Whom it May Concern.' This tells me up front that you have done absolutely no research about who we are, what we want, and what we need.