My wife seemed to think that Kryptonite had made Jason more powerful. Remember, Lex held some up to his face and he seemed unaffected. Then, when he grew a giant kryptonite island, Jason suddenly was able to throw pianos and use telescopic vision. Maybe an unlikely scenario, but interesting.
Well, here we see the problem with the film. A lot of things are never explained, so we're left to guess. Usually it helps when actors reveal things through their facial expressions, but we've got a fairly wooden crowd here (except Routh, who I really like), so we're up a stump.
Is Jason affected by the Kryptonite? What is that look on his face? Pain? Fear? Curiosity? He doesn't writhe in agony, but is he supposed to be emoting a certain degree of discomfort (as a half-human, maybe it doesn't hurt him as much)? I can't tell (Luthor seems undecided as well). Similarly, I think there are number of places where a visual cue from Lois might have helped me along, but I can't read her, either. Whether this is because of the limited range of the actors or because Singer told them "don't give it away too soon," I don't know, but when it's all over I still don't know what to think.
Here are some more points to ponder, though. How would Lois not recognize the father of her child when Clark comes back to work?
Well, this is just another argument for leaving Superman II out of the equation altogether. Either Lois recovers from the Lester mind-wipe or the whole sex act is undone by Donner's Earth-spinning. If in either scenario she somehow remembers the sex, she MUST remember the secret ID, since in both cases, Superman reveals it before sleeping with her.
Therefore, Superman Returns, if it fits AT ALL into previous continuity, is a sequel to Superman: The Movie, and Superman II -- in either form -- never happened. Sometime after the end of the first film, Superman and Lois slept together and we didn't see it.
Actually, I prefer that to watching them get it on in Superman II. It would have been more rewarding dramatically to have us suddenly realize "wow, Superman and Lois DID IT!" than to actually watch them get funky in that Harlequin Romance bed scene in the second film.