Like Louis Armstrong said about jazz, "If I gotta explain it to ya, ya won't never get it!"
For me, getting onto anybody's so called, world XI is a bare minimum. World XI teams, as a rule, are usually packed with media favourites. Nowadays, they seem to be picked on the strentgh of a guy's boot contract & image rights, rather than his footballing ability.
As others have said before me, the ability to change a game, run a game, be the abiding memory that one takes from a game. The sort of guy that after he does something so special, you're left thinking "now, somebody wipe down that ball and hand it back to the nice man...". A guy that you can name in the same breath as Pele, Maradona, Cryuff, Beckenbaur, Maldini, Zidane... It's a very, very small club, in my mind.
And (this is the crucial thing for me)... consistency. Above all, consistency. To do all of the above and do it, year-in, year out. Then you can start to think about calling a player 'world class'.
Failing that, anybody that Jamie Redknapp/ Paul Merson/ Andy Townsend is excited about, at any given time, is obviously, automatically world class, Jeff.
Also, the best player at Manyoo, at any given time, is (this goes without saying, doesn't it?) "The Best Player In The World!!!"(TM). Be it, Robson, Keane, Beckham, Cantona, Ronaldo, Rooney... all the bestest ever, ever, ever.
Great topic, by the way. Well done the OP.