The way I define World Championship is by the following criteria (I think this is fair).
It must be considered a World Championship
It must be active for an accumulation total of at least 10 years and follow a continuous lineage (ECW championship ran from 1993-2001 = 8 years then from 2006-2010 = 4 years, therefore 12 years in total).
It must be active in at least one Internationally recognised promotion (eg, WWE, WCW, NJPW, etc)
Currently, the only championships in the World that I consider "True World Championships" are...
WWE Championship
NWA Championship
IWGP Heavyweight Championship
AJPW Triple Crown Heavyweight Championship
GHC Heavyweight Championship
CMLL World Heavyweight Championship
In 2012, the WWE World Heavyweight will become a true World Championship. The TNA World Heavyweight championship on the other hand won't become "true" until 2017 (should TNA last that long

).
In 2012, you can add the following to the recognised champions list...
Kane (WWE, ECW and World Heavyweight Championships)
Christian (NWA, ECW and World Heavyweight Championships)
Chris Jericho (WCW, WWE and World Heavyweight Championships)
CM Punk (ECW, World Heavyweight and WWE Championships)