The team that is most likely to cause a few upsets would have to be Samoa. Their squad includes plenty of talented backs and forwards, however the only thing which might hamper their progress is their lack of talent in the halves. The suggestion that Nigel Vagana play at half-back is ludricous, he is already hopeless at centre which was why Cronulla fans were so happy when he left for Souths.
However much I want Papua New Guniea to upset one of the big 3 nations in the pool rounds the chances are small due to the wealth of NRL and Super League talent that Australia, New Zealand and England have to draw upon compared to amount PNG does.
France are might put up a conisderable challenge if they make the semis but to get there they will have to not only beat Fiji and Scotland to finish first in their pool and then probably beat either Samoa or Tonga who will probably finish top of the other pool.
Tonga although having a strong squad will lose a few of their players to Australia and New Zealand, and like Ireland will have to put up a fight to beat Samoa and progress to the next round.
Fiji and Scotland are in my opinion the two weakest of the nations competing in the World Cup as they have the least NRL and Super League players to draw upon, however stranger things have happened and anythong is possible in a World Cup as shown by Ireland beating Pakistan and progressing to the finals in the last Cricket World Cup.
Overall the only nations which can even remotely challenge Australia for the title of World Champions would have to be England or New Zealand. Although Samoa or France may make it to the Semis the odds against them beating Australia are pretty high.