^ Pretty much this. The thing we want to prevent is people filling up their posts with unnecessary decorating elements that would stretch out the post or make it difficult/annoying for others to read.
Adding even a single unrelated symbol (e.g. ~♥) at the start of each post is borderline, so anything beyond that is almost certainly against the 'decorative spam' rule. Using a chatroom format is definitely against the rules, as is adding things like °º¤ø,¸¸,ø¤º°`°º¤ø,¸,ø or ASCII art (for example, that Skrillex ASCII art that's around, or the running horse one). Including an in-post signature (e.g. ~ Atwood) is less concrete, since some people share accounts and therefore will sign their posts to show which person has written it, but it should be avoided if there isn't a reason for doing it. Basically, if there's no reason for it to be there, it shouldn't be there.
Text emoticons that relate to your post's content are okay, so long as they don't get excessive. 'Excessive' is going to be a bit subjective, but as an example, something like this would be okay but pushing the limit:
"O_O... e_e... >w<... >:C... (╯°□°)╯︵ ┻━┻ " (this is comparable to something like '

')
while this would be over the limit:
"O_O O_O O_O O_O O_O O_O... e_e e_e e_e e_e e_e... >w< >w< >w<... >:C >:C >:C >:C... (╯°□°)╯︵ ┻━┻ "
just like its emoticon image counterpart:

As with the emoticon images, you only need a few to get the point across, so repeating them on and on has no purpose and adds nothing to the post's content.