The horizontal slats of wood.
The use of fkagstone
The beds have one species, so as not take away from the focus; the trees and flagstone pathway. I also like how the pathway flows.
A great detail to separate mulch from the concrete pavers is to insert a vertical border with large pebbles beside it.
I like the use of the trees (colour & form) in this narrow space with the screening fence as a backdrop. That's a tough space to make interesting and you've executed it so well