Hate to be pedantic here but I think World class would just be defined as being amongst the best in the world, so realistically it probably means being amongst the top 5 or 10 players in your position in the world. It is also entirely subjective and entirely in the eye of the person doing this.
Who do we have tevez yes (I would say except Spain he would get into any side in the world pretty much), Given I think is world class or close, Barry looks it for England but not us and i think Nige is close to one of the top 5 defensive midfielders in the world. Robinho is certainly world class when used properly and for brazil he is consitently world class. then we have others who have the potential to be world class or can be world class on their day (Ade is someone who is virtually unplayable when on his game but is not consistent enough)
Of the players we are looking to sign Ibrahimovic, Silva, Toure, Alves, torres, gerard etc are all clearly world class players!