When authors follow instructions.
Most notably, submission instructions. I get plenty of submissions directly in my personal email, and guess what - just I like I state in the right sidebar on my blog, I delete them (after making sure they don't say "requested" or "conference" or "referral" anywhere of course). I am always shocked, too, how many people submit to my query box after a conference...when I give them my card and say to send directly to me!
I get it. There's lots of rules. Lots of things can go wrong. I don't hold it against anyone when they don't do things exactly right; but most absolutely do you stand out when you do. Or at least not inspire a frowny face. You don't want your submission to inspire a frowny face, even if that isn't held against you, right?
Competing with critique groups.
I think critique groups are incredibly important for polishing up your manuscript before you send to your agent or on submission. But I'll be blunt: I don't give a fig what your critique group tells you about your manuscript. If I'm telling you "this has got to go, I can't sell it like this" I absolutely hate hearing back "well my critique group says they think it's fine."
I'm not saying by any means that my word is final; I'm always open to collaboration on edits. There've been plenty of times an author has come back to me and said "well, I'd like to keep this for xyz reason." Because of course, if there IS a reason, it may be that other parts of the manuscript need to be tweaked in order to understand it. But that is a conversation I want to have with you, as the writer - not with your critique group!
In other words: don't use your critique group's "voice" as an excuse not to make an edit. If you don't like a suggestion, explain it in your own words. And if you trust your critique group's judgement on the marketability and edits needed for your manuscript above your agent's...well, there's a bigger problem to address: why?
But that's a can of worms for another post. ;)