A long time ago I tore a branch off a tree, barked it and finished it with a few coats of varnish. It was more fun than functional. The stick was not that straight so pressure from using it caused it to bow while walking. I still have it but it made me wonder. 

What do I need to consider when selecting wood for a walking stick? I have heard of people using saplings, roots and just branches in general. 

I am not interested in answers that are just favoring species e.g. diamond willow etc. but something of a checklist when it comes to selecting wood.