I begin this review by confessing my bias against films in which there is fundamentally no likable or redeeming quality in its main character. I recognize that this doesn't disqualify Raging Bull as a bad film--and at a 95% Fresh rating, Rotten Tomatoes would affirm that--I just didn't enjoy it.
Robert De Niro did such a superb job depicting the bullish, jealous, gluttonous, and at times savage qualities of Jake LaMotta that I found myself never willing to commit a care to the film. This wasn't a rise-and-fall story of a man you're meant to root for--at least, I hope not. The real LaMotta was pretty damaged guy, rapist, and a domestic abuser. And though in the film, LaMotta fell constantly victim to the Mob and the press, this wasn't enough to ever earn my sympathy.
In fact, it is only by looking at Jake's brother, Joey LaMotta (brilliantly played by Joe Pesci), that I was able to avoid growing bored with the film. It was Joey's story I found fascinating: A younger man who held the moral high-ground over his brother (as depicted through some spot on religious imagery) and his attempts to rein in his brother's testosterone-laden, damaged-brain decisions and the cost of his own bad choices.
Again, I suppose I see why this film is heralded as a "modern classic" and Scorsese's 'magnum opus'. And I can handle a tragedy. It just wasn't for me. Ironically, Scorsese then-and-now dislikes boxing (considering it to be boring and brutal) and only found interest in the story when he considered it as "an allegory for whatever you do in life". I suppose I get that it can feel like the world is against you, but still: you don't have to be an ass about it--or a raging bull.