How on the District line tube trains you can see all the screws and rivets and see how everything has been put together.
How no-one writes the letter 'a' how it appears in printed form, yet we use it and associate it with that letter anyway.
Old signs that are still visible even when they're not relevant anymore

If you cross your eyes on a repeating pattern so that pattern meets up, you get an odd 3D effect.
How you can tell the likelihood of rain by the types of clouds in the sky.
What sort of things do you notice or enjoy that are overlooked by others?