The NN&Greg Guide helps new comers... sort of. They explain everything in such great detail -- that most of it's not even necessary. KISS: Keep it simple, silly!
As for what Juan wrote:
Also use almost perfect grammar and use really neat words.
Bad Examples
Good,Funny,Nice,Cool
Good examples
Amazing,Hilarious,Majestic,Entertaining
Although adjectives like that may jazz up your potentially merit-worthy comment, using simple - concrete words would actually help make your point much clearer. Good, funny, nice and cool are actually good examples of words to use in your comment (of course, expand on them and provide supporting points).
Also, most people say: "Hey, ADD BIG FANCY WORDS" -- just don't. To the contrary, big fancy words take away from the clarity and meaning to your comment. Similar to the above paragraph, just... just keep it simple, concise and clear. No need to write: "This game was majestic in all it's sipidness and it was exuberate!" when you could just write: "This game was fantastic, because..." :)