I don't think it necessarily matters if it's not displayed throughout the lesson, but it does need to be shared and explicit. Do students know what the point of what they're doing? If the observer isn't clear, the students probably aren't either.
This is probably your AfL: a measure of what has been learned up to that point and, importantly, to what extent. It will need to be revisited if it's to stick and accumulate. It doesn't have to be an overly-conscious and arbitrary display for the observer but it's very important to gauge this before learning can move on, either in this lesson or the next.
Very often it's easy to ignore this, especially if there are few behaviour issues. But if there are, that's because it's well-managed and good habits have been established. By consciously looking for these habits and for signs that measures are in place when someone raises their head, it's easier to pick apart successful behaviour management.
I like this one because the teacher may not always realise that one or two students either dominate or slip off the radar. Is any one student's name used more than others? Why is this? And in the discussion afterwards, look at the students who might have needed additional support. Were they supported sufficiently in the lesson? The observer doesn't need to pass judgement on this though; it's a starting point for discussion.
These last three are taken from the developmental Learning Walk process I learned about on SLE training, detailed here. They seem on first glance a bit wishy washy, but after using them a lot in the past 2 years, I’ve seen how effective they can be.
Who would't like being told their lesson contains something that's been favourited? Observers can include resources and displays here, as well as relationships, tasks or anything else they liked.
This is often the hardest to articulate. But how did the lesson feel? What was the atmosphere? Quiet and studious? Quiet and lethargic? Buzzy and exciting? Buzzy and out of control? Calm? Safe? Too safe?
This is maybe the area that would normally count as the target area, but because it relies on questions rather than judgements, it invites reflection. As I said in more detail in my Walk On Through post (linked above), this part offers the teacher the chance to justify their teaching choices. Sometimes there's an answer; it just wasn't made clearer in the lesson. And sometimes there's not, which can lead to a greater understanding of the process of teaching and learning. But it also allows explanations to be made that the observer just wouldn't have prior knowledge about, especially perhaps when thinking about differentiation (see also my post here). Much of the resentment that arises from classroom observations involves the teacher feeling they didn't have a voice or the chance to explain themselves. This process allows that opportunity.
Quick extra thought:
What about if the observer didn't write anything in the lesson and just absorbed what they saw? Following the lesson, the observer and teacher could have a go at filling the progress form in themselves for 10 minutes, without consulting. Then compare and discuss. It would be very interesting to compare perceptions of the lesson from the observer's (and students' if they've been spoken to) with the teacher's. I'll definitely try this out in the next few weeks.