I came from the same RP MUD that @Arkandel did... 20-odd years ago. I remember a time when two-line poses were awesomely wordy, and RP through socials was the norm. Thankfully, my style (and that of most people I interact with) has improved since then.
After that, there was a character-limit (350ish, I think) on poses. That was interesting. But really, I think that a pose should be as long as it needs to be in order to get across the words, the actions, the emotions behind them, and the character behind them.
Yes, that's a wishy-washy answer, I don't care. I like to make sure that all of my characters pose differently, at least to some degree. Whether it's an accent (don't kill me, I haven't really done it in years now), particular word choices, different curses, this is easy to do with what the character says, but I think it's equally important that the rest of the pose be unique to the character also.
For example, my character Gray from The 100 was a bit of an asshole, and frequently made bad choices and assumptions, so I snarked like hell at him in my poses. Dog, from Generations of Darkness, was so loyal that it hurt (what do you expect, he was a clone trooper), and so I did like to mention when he was being naive or flat-out wrong in my poses, but I didn't snark as much. Termiane, from Cuendillar, was more of a wish fulfillment character (I'll admit it this much later), so I was a lot more serious with his poses.
I have definitely come to prefer either a single paragraph of text in a pose, or just single carriage returns between paragraphs if you've really got to be wordy or are NPCing multiple people, but I prefer not to have multiple line breaks in someone else's pose, because I think it looks messy in a log. I very much prefer it when people put wikicode in their poses--makes things a lot easier on me, since I usually take care of the log.
As several people mentioned: god please don't pose in the second person, it's creepy. Also, please pose present tense, not past or future.
As for providing a look into my character's head, I agree with several of the other posters -- I want to make sure that my character's intentions are clear (if they're supposed to be clear), but I also don't like putting too much of my character's mind into my poses, because I would prefer to put in gestures and expressions.
Most importantly, however, I want the people I'm RPing with to respond to the hooks I put out there, and to offer hooks for me to respond to in turn.