When I write something, I like to be as detailed as necessary to fully convey my message, and that often turns it into a 750 word essay. I'll just be typing and writing down my thoughts, and then I look at the screen, and I have four paragraphs.
If I see a long post, I expect it to have at least acceptable grammar and a clear reason for it's existence. It can't be messy, disorganized, and difficult to read. I can read the first paragraph or two of the post and skim over the rest of it, then deem whether or not I feel like reading it.
Here's a guideline that I have found myself to unconsciously follow often:
If it is a long post by someone that I know can make a good post, then I'll read it. If I'm not sure, then I'll preview it before diving into the post.
An example of a good post that isn't long is those that belong to DPinky and RC_4777.
I've found DPinky's posts to carry short but powerful messages sometimes. He doesn't usually write much, and I've noticed that his sentences don't always make sense, but when he posts on a serious topic, he often has a great deal of insight.
However, there are also people such as BitoBain and Miner9823 who share so much information and put so much detail that it simply requires a 3-page post in order to fit everything and also maintain their desired format.