They disabled the right-click-and-select-open feature; you now have to go to Settings to run the app.

Nowadays (as of Sequoia, I think), I find that I need to run `xattr -c Foo.app` to clear the “this was downloaded from the Internet” bit on the application bundle before I can right-click, “Open” it. Used to be that you only needed to do that with .apps extracted from zip archives, but it seems to apply to .apps copied out of disk images (DMGs) now, too.