I think what separates actors of yesterday is that they could actually act; a lot of them seem to play themselves but in a fantasy role. Take all the different parts that actresses such as Bette Davis and Joan Crawford, Yul Brynner, James Stewart, Gregory Peck - they could play comedy, villains, angelic roles and play them all perfectly. When I see today's actors, they don't really differ in their ways (if you know what I mean) than the characters that they then become famous for (I can't imagine Tom Cruise or Bruce Willis straying too far from their usual roles).
Although, to give praise to one current actor, I do think that Christopher Walken can turn his hand to most roles whatever they call for so there are some that still deserve the title.