One good thought experiment to sort the Mario Chalmers from the Chris Boshes on any given roster is to look at the nominal starting five and sub them out, one at a time, for LeBron James (or an equivalent franchise-altering superstar if you don't want to envision James on the Celtics).
So, looking at our nominal depth chart at ESPN:
espn.go.com/nba/team/depth/_/name/bos/boston-celtics
We're starting
Zeller
Johnson
Turner
Bradley
Smart
(the Lee trade isn't there yet)
How much better would our team be if it was
James
Johnson
Turner
Bradley
Smart
or
Zeller
James
Turner
Bradley
Smart
or
Zeller
Johnson
James
Bradley
Smart
Not appropriating for position, just a sheer upgrade in talent -- and you can see which players are, relatively speaking, more or less important than the others, at least in terms of being better for the squad on an individual talent level than their contemporaries by virtue of their hypothetical, vacuumed replacement.