I recently had to use a computer with OS X Mavericks on it (10.9 I think). I was struck by how beautiful the interface was. I'm having trouble finding a good screenshot illustrating this, but compare
http://content.gcflearnfree.org/topics/243/2013_new_desktop....
http://download.softwsp.com/sites/12/2015/11/os-x-el-capitan...
Everything just looks better on Mavericks. The gradients may be over-the-top but they're at least consistent. Transparency on El Capitan is pointless and ugly. Maybe I like the system font a little better.
Firefox is also a really good example; it looked great on Mavericks but has not been able to fit in since.
Usually, I prefer simple UIs: i3, terminals, etc.. But the look they have done for Yosemite/El Capitan just doesn't work.