I think De Niro wasn't used as much as Liotta was obviously the main guy. And after that I'd say Pesci was the second main character; his hotheaded-ness ultimately led to his own whacking, and thus to Henry's defection.
De Niro is like that, though; he was the perfect main cast for Casino, and the perfect backdrop in Goodfellas to Liotta and Pesci.
Mick