Although Rocky is constantly maligned today, but I do enjoy the film myself.
Stallone is someone today who most people would think would have never ever been nominated for an Oscar. But he was and that was not the feeling at the time at all. He was heavily praised for this performance even being compared to Marlon Brando, but I image they just did not know him well enough yet.
Stallone is a performance that I do not hate in fact I enjoy his performance to a degree, although I cannot defend it exactly as great acting, though it certainly works for the character of Rocky. Stallone is good and seems dim witted enough as Rocky. I think is fine as the hard headed boxer persona, but he has obvious problems when it comes to doing more than this. He is fine when he is just acting stupid and trying to attempt to stand up for himself.
Although I'll say they overwork the way she's made up Stallone does strike up some chemistry with Talia Shire as Adrian. It is a simple though effective sort of way as the two people broken in a certain way comes together. Stallone does well to portray Rocky's slowness as almost a way of being able to easily deal with her own shyness, since he's a man who frankly does not mind waiting, and knows exactly how it is that he should maneuver around her. When Adrian breaks out of her shell there is a genuine warmth and sweetness when they are together that work well.
Stallone perhaps later on proved he was not Marlon Brando as he did not prove to have a great deal of range later on. Nevertheless Stallone simply is Rocky for a reason. You can't see anyone else in his role as the certain personality of the man is definitely well realized by Stallone and he makes a truly endearing character out of this. You want him to succeed and perhaps it might not be a great performance, it is a good one that is perhaps too often derided.