Trifecta of Rejection

The trifecta of rejection is different for every editor. Many factors come into play. Is the manuscript something I acquire? Do I like the concept? Is the query put together well? Those are all about first impressions for me. But when it comes to opening the sample pages, there are a few basic things that can make me reject a manuscript. The trifecta of rejection for me is the following:

Passive writing

Was, were, had. Sometimes it's necessary to use it, more often, it is not. The rule of thumb for me is: the less you use it, the better. It takes the reader out of the action, the here and now.
Broken/Incomplete sentences
True, some feel it is "voice" or "style", I am not among them. If you fancy using incomplete sentences, I am not the editor for you. When used infrequently, they can be good, powerful, but when overused as is common, they lose that impact. If you sign with me, expect to have 99% of your incomplete sentences edited out.
Excessive Adverbs or Adjectives
Particularly as tags on the end of dialogue. Don't do this. It's lazy. Rather than modify or qualify your adverb or adjective to express a relation of place, time, circumstance, manner, or degree, it is better to use a more powerful adverb or adjective in the first place. Or, you need better phrasing or lead up to it. 

The trifecta will likely be different for every editor. The best thing to do is make sure you have edited your manuscript thoroughly to the best of your abilities, and do your research on who you are submitting to so you make sure they are right for you and your work.