This is a great video on level design that is very relevant to working in TF2. Highly recommend it.
View: https://www.youtube.com/watch?v=GZ99gAb4T0o
The pathing examples felt weak as the map itself is incredibly simple to begin with.
New video came out today well reflects more on level design programs (includes hammer!!!)
It's also good to mention that players and their models are generally noisy, so less noisy materials like smooth concrete or planks (and of course dev textures) help make players and objectives stand out more.