While Drupal 8 has plenty of things to be excited about, there are a few "gotchas" that site-builders need to be aware of as they build out sites. I found the first thing that I had trouble with was the way that Drupal 8 isn't very consistent (yet?) with the way it handles paths. In Drupal 7 and before, anytime you needed to enter a path, it (almost?) never started with a leading "/". For example, need to add an new alias for a node? You would enter "my-new-node", not "/my-new-node".
With Drupal 8, a leading slash is required for path aliases. Unfortunately, leading slashes are not required everywhere. For example, in Drupal 8 page displays in Views do not require a leading slash. Neither do contact form post-submit redirects (they actually require something like "entity:node/743"!) But, block visibility settings require the leading slash. In most cases, the help text indicates when the leading slash is necessary, so it helps to pay attention!