I do have three suggestions:
1) I think you could stand to hold to a second or two more of the action inside the bus as it's rolling--it looks pretty real, which offsets the kind of unrealistic effect of it being initially thrown.
2) When Ursa and Non are lifting up the bus, you see Ursa lifting first, and the bus looks pretty horizontal, then a look elsewhere, then a look at Non, who only then begins to lift up his end of the bus. Could you reedit that a bit so that it looks like they're lifting simultaneously?
3) When Superman says "No, don't do it! etc..." After he says "No!", he gets this sort of weird half-smirk on his face, as if Reeve feels that the line is kinda corny and can't keep from laughing...is there any way to cut around that?
This isn't in any way a critique of your work--you do a really could job. It's just a tricky scene, that due to the nature of sfx at the time as well as some kinda crappy acting on Reeve's part in that one scene, has some really distracting clunky bits. So...whatever magic you can work would help.
BTW, what software are you using to your editing and special effects?