With threads that aren't super long, I read the whole thing before I post (say, six pages or less). Any longer than that, though, and I'll either skim read, or read just the first and last two or three pages, or just not bother replying at all. I've gotten interested in a thread in the morning, not been able to get back to it for a couple of hours, and then come back to find there are fifteen pages of posts already. I try to catch up with the first and last pages, but reading the whole thread is not usually an option.
As for missing details in the OP, I tend to skip lines in very long posts. I've been this kind of reader my entire life; it's a great habit for redundant textbooks with lots of example problems or situations, but not such a great habit for everyday reading. I don't do it with really engaging novels and the like, but even with fairly interesting reads (novels, posts, anything), I have to constantly work not to skip whole sections. I promise I don't do it on purpose. It's just as annoying to me as it is to you, I promise.