Posts should only be written in a narrative format, not in chatplay/asterisk roleplay format. It's highly suggested that you also write in 3rd person perspective.
An example of "chatplay" would be:
Character: I LOVE! *he TURTS around* :Y
An example of (3rd person) narrative roleplay would be:
The character turned around, "I love." he murmured.
Now, concerning the length of these posts, we'll be a bit more lenient. Now, you can always write up the occasional one liner, but it's preferred that it's at least two lines long. This simply keeps things going, as well as interesting. Paragraphs are wonderful.
Also, disregarding the length of the post, your posts should also be thoughtful. Try to give more information, rather than just blatantly stating actions. Try to show us how a character is feeling, thinking, etcetera. It certainly helps to spice things up a bit and help express your character better.
Ah, what might "roleplay crimes" be? Well, to put it simply, things you should probably never do when roleplaying. I've compiled a list of them.
Meta-gaming basically means that your character knows something before others may have known it. Whether it be reading thoughts, or just knowing where certain people are, it's excruciatingly bothersome and to be avoided.
God-modding is when a character has the ability to do basically anything without consent of others, or boundaries. This one should definitely be avoided at all costs.
Excessively writing your character out-of-character writing: Now, this one is more understandable, considering that characters are dynamic and constantly changing. However, if you strictly state that your character is always "ANGRY" and always play said "ANGRY" character as if they're always happy, then you should probably revise a few things.
Also, refrain from making your character act OOC for the sake of "the ship" (a relationship). Yes, people may actually act a bit 'off' if they have a crush on a certain someone, but that doesn't necessarily mean their entire personality changes. If you have a detailed reason for why they may act extremely differently, then we may let it slide, but otherwise than that please don't do this.