there's a serious tonal difference between the first film in the series and the sequels that came after. the first film is almost a character study, while the sequels move forward in a much more spectacle-driven way.
but that innocence, that honesty at rocky's core never gets lost. in my opinion this consistency is exactly what's kept the series standing for so long. no matter how famous the character gets, at his core he stays the same neighborhood guy, and the audience feels that. so rocky is proof of what a well-written character is capable of.