You know -and this is a silly thing to get all annoyed about, but- I hate it when people write something (fanfic or anything that's supposed to have that "professional" feel to it) and don't capitalize proper nouns.
For instance: Dad
versus my dad.
The first one, "Dad" is a proper noun. It is a title bestowed upon a person, so when you have dialogue and the character says, "Yeah, Dad went over to the lake," it's the same as writing "Yeah, John went over to the lake." You wouldn't write John in lower case because someone else knows him by the name Cookie, just like you wouldn't write Dad in lower case because the character being spoken to knows him as John.
When a character says "my dad," they aren't using a proper noun; just a regular one. It's nondescript. Maybe they call their father Papa, or Pops or even by his name, John. The addition of the possessive noun "my" implies that the noun following it is usually just a regular noun; my car, my boat, my house, my balloon. It's considered a BFD when someone says something like, "my John," like driving home the point of ownership. It's not so much a grammatical change as it is a social cue.
That's why when people don't capitalize God when using the word as a proper noun, I get annoyed. It's the same as not capitalizing dad or john or cookie. It's awkward. If you use the word as a proper noun, capitalize it. We learn this in elementary school. "Oh my god," is cool, because the possessive is in front of it thus implying that the noun following isn't a proper one.
Capitalizing God doesn't suddenly imply that you're a believer or that you hold him in reverence; it's actually when you start to capitalize things like Him and His Will and stuff like that, that's when people should clue in that maybe when you capitalize God, you mean it out of respect.
Every other time, it's because you know the basic rules of English.
So when someone writes something like this:
Castiel turned to Dean, eyes narrowed in annoyance. "No, Dean, god is real."
-that really irritates me. It's a proper noun. It is the name of a character in the story.