I fucking hate long posts. Why don't people realize that I don't want to read the whole post. They should just get the point in a short post. Really, it's annoying as hell. There was a long post on this other forum and this guy was like "(insert long post here)" and it was like 3000 words or something. No one wants to read that. Yet all the replies were (of course) long too. This Minecraft server once when I walked in was all like "Hey dude, help me grief" and I was like "wtf...?" and he was like "yeah" and it was fun. But when people post long posts they are just wasting their time. I don't even know why people read the posts.
I also hate when people go off-topic in a post. It's also annoying. They are just talking about one thing then all the sudden "lol (insert something off-topic here)".
But what I hate the most is when people talk about something way different than what the topic title is. If you talk about something different then you should just change the topic title.

If they post short post, admin and mods will consider that as spam.

Long posts are better and short posts like 2 sentences is like spam.

I prefer long posts, but it's helpful if they organize the post.
Hitting the enter key every now and then can help significantly, it makes all that text a lot easier on the eyes.

I definitely feel like all posts made to start a new topic should be at least three or four sentences, unless it's in the spam forum or if they have other things to offer to lengthen the first post.