the cube comparison thread has an "the average 360 cube plays" section, and i think that's a good place to start. it also has a simple ranking of each card, so you can see the top cards in each color.
When I see Healing Salve, I'm often like "Oh girl, I wish I could turn every card into this." Thanks they removed the gain life part, otherwise this would have been broken.
I think that staples are anything run by more than a certain % of the cubes listed. 66% seems like a fairly good cutoff point.
So, since currently there are 72 cubes, that comes down to all the cards that appear 48 or more times in the breakdown. And with a brief visual inspection, that looks about right.
There are heaps of cubeworthy cards below that cutoff of course, it' just that you start to see the cards that people have quite definite differing opinions about at that point.
The cube comparison thread is a good place to start, but don't take the rankings as gospel, as it is dependent on cards that people actually have (for example, Great Sable Stag is in more cubes than Survival of the Fittest). I would make sure to take a glance at the PRE rankings (which are currently being updated).
http://forums.mtgsalvation.com/showthread.php?t=271371
So, since currently there are 72 cubes, that comes down to all the cards that appear 48 or more times in the breakdown. And with a brief visual inspection, that looks about right.
There are heaps of cubeworthy cards below that cutoff of course, it' just that you start to see the cards that people have quite definite differing opinions about at that point.
http://forums.mtgsalvation.com/showthread.php?t=221992