Some have been mentioned,
Stu Kelly eels to broncos, went from contender fo origin centre spot, to qld cup player. Ended up as a solid winger no more.
Muir to nth sydney, was rated close to world's best forward was never the same.
Corey Pearson to parra, had been a good to solid player at previous clubs, but was bad at the eels.
Whoever said Hodges to the roosters, thats crazy, he went from broncos reserve grade to rep star. He then went backwards, but the initial couple of years were great.
Chris Thorman to eels, went from possible GB player, to reserve grader.
Paul Green after he left sharks was never the same, same with Noel Goldthorpe after he left saints.
Brett Plowman after he left brisbane, for those who don't remember him he was a giant winger who would come into the side when their rep stars were out, looked awesome, but doing it week in week out was a problem.
The usual case is guy who looks good in a good side, moves to weaker club, and you realise the players around him made him look good.