Kaka - known story
Arshavin - looked immense when he joined Arsenal and I thought he'll be exellent for them but after all he was quite underwhelming
Torres - known story
Diego - looked like a future world beater for Bremen. Was absolutely sure he'd be among the best in the world sooner than later when he left but he never showed his quality after that
van der Vaart - similar to Diego really but a bit less extreme
Fabregas - was perceived as the greatest Spanish talent in ages and was brillant for Arsenal, then average for Barca and nowhere near the former player for Chelsea. He declined aged 20 something.
Götze - he can still turn it around but from being one of the best players in the world already as an U20 player to a decent Bundesliga player at 24+ is an extreme case
Some others from the top of my mind: Sahin, Kagawa, Thomas Müller, Adriano, Pato
Arshavin - looked immense when he joined Arsenal and I thought he'll be exellent for them but after all he was quite underwhelming
Torres - known story
Diego - looked like a future world beater for Bremen. Was absolutely sure he'd be among the best in the world sooner than later when he left but he never showed his quality after that
van der Vaart - similar to Diego really but a bit less extreme
Fabregas - was perceived as the greatest Spanish talent in ages and was brillant for Arsenal, then average for Barca and nowhere near the former player for Chelsea. He declined aged 20 something.
Götze - he can still turn it around but from being one of the best players in the world already as an U20 player to a decent Bundesliga player at 24+ is an extreme case
Some others from the top of my mind: Sahin, Kagawa, Thomas Müller, Adriano, Pato