Like when he's talking about Pa Kent or when he's talking to Jor-El on the ship and basically throughout the entire film when he isn't yelling he's just got this same bored monotone delivery. His performance isn't matching the words coming out of him much of the time. And I thought he was positively awful in the final scene at the Daily Planet. He doesn't even try to hide his identity. Compare that Christopher Reeve who completely transforms when he puts on the glasses. Voice, appearance, everything. Reeve's scene in the first Superman film in Lois Lane's apartment when he swaps back and forth between the two is one of the best acted scenes of any superhero film and here I couldn't by it at all.
I watched Superman: The Movie again before watching this and maybe it was a mistake because it made the decline in quality pretty much across the board that much more noticeable.
I find some of the other comments here about the acting interesting, because the three performances that I thought were better in this go around were the ones that seem to be getting the most criticism, namely the three main women, Lois, Martha, and Lara. In particular, this was the first time in really any iteration that I actually liked Lois. One of my number one pet peeves in cinema (or television or literature) is the dumb character who is supposed to be smart and that's what she is most of the time. This Lois was actually intelligent, tough, and resourceful for once and I bought it. Bosworth was completely underqualified for the role she was playing the last film, and while Kidder did the job and was energetic I also got the sense that her Lois had a few loose screws if you catch my drift.
Shannon was fine, but didn't leave much of an impression on me. He was certainly no Terence Stamp.
Crowe and Costner give the films best performances, but I can't really say that either were better than their counterparts in the Donner film. I'll need to see it again to get a better idea of where Costner compares with Ford, but as good as Crowe was, Brando gave one of my favorite performances ever in the original stealing every scene he was in which Crowe doesn't quite do here.