I've talked about it before, but I wanted to revisit the issue of the delete button. I recently joined an awesome forum for working on queries and one thing I'm noticing is a tendency toward verbosity. Put simply, people use too many words to express a simple idea. This is a problem literature has always encountered (60 pages on gutting a whale, seriously?), and as someone who studied the early novels of the 18th century, I can tell you that editors and agents have really helped in this regard.
The most common mistake I see is people writing a complex sentence for a simple idea. People do this for many reasons. In my past life as a Composition instructor I found many of my students were trying to mimic more complicated sentence structures to sound more formal. While it's important to understand how to construct sentences, don't undervalue a simple statement! Don't use 20 words when ten will do in your queries or your manuscripts. Other times the author is using passive voice, which adds wordiness to the sentence. And still other times, the author is telling us about the book or analyzing it (if you see heart-warming or fast-paced, cut it!).
The same rules apply to MSS and queries
1. Be succinct
2. Use active voice
3. Show don't tell