Given the ubiquity of 90-degree angles in Minecraft, it can be easy for things to feel a bit flat. One way to make things stand out is to use depth in your walls. Build them across a couple of blocks, with beams sticking out across the front. Or use stairs, walls, and slabs to mix things up – both are great ways to make things a bit more exciting.
Like variation, this ultimately depends on the build, and will probably require some trial and error. But when done right, a wall with a bit of depth is leagues ahead of its flat counterparts.