"Sweet music and timelapse". That's what I think of when I consider demo reels.
This is more or less true. Everyone like seeing timelapse. Sure, it may be overused, but to me, it will always look good. It shows film's ability to capture something that's impossible to see with your own eyes (although if you can, please teach me). It establishes on your reel that you're a cinematographer that not only knows how to do a timelapse, but more importantly it shows that you can make something that no other artistic medium can.
But, of course, that's not all there is to it. You can have the best timelapse in the world and still have an unmemorable reel. It's overused nature is its greatest weakness. Since timelapses (timelapsees?) are arguably a staple in demo reels, something else needs to be in there that shows the cinematographer's creative style, namely everything else.
This everything else really depends on what the cinematographer like to shoot and feels represents their work as a whole. Most of the reels that I've seen go with one of two things. Either they mostly keep the shots wide, showing breathtaking vast environments, or they get in really close and show attention to detail with things like ECUs of people's faces or rack focusing on objects in the frame's foreground and background.
But really, there doesn't seem to be any real hard and fast rules. A good demo reel can incorporate anything as long as the cinematographer feels it's representative of them. And as long as it's obvious that there's passion behind it, it can be anything.