In Yosemite, opening folders in a new window by default is extremely difficult. You can press CMD as you click (not optimal) or right-click/option-click the folder to open in a new window (not optimal).
The solution that seems to work best for me is to hide the toolbar whenever I open / create a new folder, because folders inside windows that do not have a toolbar will always open in a new window.
But the toolbar always reappears whenever I create a new folder.
So, ideally, I would love to have it ALWAYS hidden by default, and on rare occasions when I need it (to look at external drives, or drag an item into the application folder for example), I would shortcut-make it appear, ta-daam.