Source material. Using the Donner movies, probably the widest known version of Superman in the world by general audiences (yes, this is fact), as inspiration for a story which he expressly did not want to make an origin? That works. Look, I am also in the camp that would have liked to have seen a clean restart. Bryan Singer is not. And, for general audiences, Christopher Reeve's Superman and his characterization is a lot more digestable as the definitive Superman story than what we've seen in post-Crisis. Not to say a post-Crisis movie can't work in theory, just that it can't work without re-telling the origin and re-establishing all the motivations and relationships between characters. And Singer did not want to do that...And really, as a film-maker first and fan second, can you blame him? An origin steals loads of time from the main plot in order to establish motivations, which is a lot trickier with a character like Superman, as opposed to Spider-Man or Batman who are defined by one specific moment (and look how long it still took to get to web-slinging in Spidey 1). Superman needs more explanation of his backstory, and explanation means exposition, and exposition means less time to tell your real story (in Metropolis), and less time on the real story either equals the feeling of a "split movie" (like the Donner film) or a shallow film which disregards the weaving of subplots or lacks in characterization for everyone but the one or two leads (like Daredevil and Spider-Man 1). So, as a compromise, Singer has an established Superman, and is using his return to explore his motivations while fully integrating that exposition into the plot (his reasons for leaving and returning). You just can't do that as well with origin. I think it's a great idea, personally. Don't know why some don't. Maybe it's because of the perception of this movie as a sequel? We'll get to that.
Story. This is a big one, and, similar to the source material connections above, we'll examine it on its own merits. Because, honestly, whether there is no connection to Donner, an implied connection to Donner, or an overt connection to Donner, it will not matter to audiences so long as they can understand this movie in its own context (a self-contained storyline) and the set-up is adequately explained. Then, if the story can take the viewer on a hell of a ride, from beginning to end, it will succeed. So basically, will this story of Superman's return be a good one? After all, I maintain that a good story which lives up to audience expectations is the only thing you need to sell tickets (we'll discuss audience expectations/perceptions next). Great performances get people to come back that second time, but if Spider-Man showed us anything, you can get by with stilted dialogue and some uninspired performances if you tell a compelling story. And I trust the storytelling skills of Dougherty, Harris, and Singer. In fact, I don't think anyone's disputed that. We know very little specifics about the plot at this point, but there are some good very compelling reasons why one can say this story will be succeed, beyond just that the writers are good. To discuss this, we now look at...
Audience perceptions. So, if written by good writers, will the story of Superman's return to an inhospitable world be one which grabs the interest of the mass audience? To answer that question, one need only look at the state of the world today. Do we need a Superman? **** yeah, we do. And if we can't get one in real life, we'll take one in the movies. Do we wish we could give a face to our enemy, a concrete identity, like say, Lex Luthor? I wish to God we could do that instead of chasing underground terrorist cells all over the globe. And do we, the people of the real world, need a new, inexperienced Superman coming in green to clean things up? **** no. Do we need an all-powerful, do-gooding demi-god with the motivation AND skill to clean up the world? Yes we do. And it seems obvious, so will the masses in the world of Singer's Superman. So the return of Superman, the return to this ideal of truth and justice on the largest scale, the return of the one true superhero amongst the angst and darkness which has simultaneously flooded our lives AND our movie screens...Is just appropriate. I truly believe this is an idea the public will embrace wholeheartedly, after some initial skepticism concerning the fact that this is not an origin movie. Which brings us to...
Audience expectations. The stickiest of the sticky situations with this movie. Here's where we discuss the sequel status. I won't sit here and say with any certainty that it will be a direct sequel or it won't, because I don't know, and neither do you. But I will say this: no matter the sequel status, it will not matter. As long as, as I said, this story is self-contained (much as Spidey 2's story was self-contained, beside the subplot with Harry) and the general audience can understand all events, there's no reason for them not to enjoy it. Here's why: If it IS a sequel, and the movie starts, and the movie can inform the audience of the set-up, then your STORY problems are solved (we'll get to other expectations of the audience later). Simple as that. Honestly, even if, by some WILD circumstance, this movie involved General Zod...All you have to do is include some kind of flashback (or Alex Ross drawings or whatever) showing that Zod is a deposed general of Krypton which Superman was able to defeat by draining his powers. There. Now the audience is in the know. You don't have to recap the entirety of the first two movies for people to enjoy or understand, much as you don't need to do so with Spider-Man 1 to enjoy Spidey 2. "Given Spider-powers by a genetically-altered spider, Peter becomes Spider-Man to atone for the murder of his Uncle Ben, which he blames on himself. Meanwhile, he develops an unrequited love for MJ Watson, but his best friend Harry's father, the super-villain Green Goblin, finds out his identity and tries to kill MJ. Even after the GG is accidentally killed, Peter decides he can no longer be with MJ for fear of something similar happening again." There, ater three sentences, you can now enjoy Spider-Man 2. It really is that simple. Singer will not expect the audience to come in knowing exactly what's going on (that's bad storytelling AND bad business). No one is going to have an anuerysm (sp?) because there is five minutes of important set-up at the beginning of the movie (cough cough original Star Wars cough). And I guarantee you the ties between SM1&2 are a lot tighter than Returns' will be to SMI&II. Again, it's just bad business. May there be ties? Yeah, maybe. But just give the audience the pertinent information clearly and concisely, and they're ready to enjoy your story.
"BUT!" you exclaim. "Shut up", I say. "No, seriously...But won't the audience be turned off that the film-makers just ASSUME them to know Superman's origin already? They'll be pissed off!" Well, even knowing we'll see origin flashbacks (which we now know is true w/ the 15-yr-old Clark casting), I will still say you're right, to some extent. But here is where I insert my own BUT. (Listen up, WB marketing department) There is a simple solution. In your trailers and other assorted promo materials, you include some variation of this (with my comments in parentheses): "Rocketed from the doomed planet Krypton (okay, now everybody and his/her grandma already knows you're talking about Superman), a young baby was found and raised by caring and loving humans. When he was grown, disguised as mild-mannered Clark Kent, he fought a never-ending battle against the forces of evil (coupled with imagery evocative of the Reeve movies, if not exact footage from them, everyone is now clued in to the basic premise). He became the world's greatest hero...Until his most bitter enemy drove him away (insert shot of devious-looking Kevin Spacey with distinctive bald head...Now everyone goes: "Oh, Lex Luthor! Oh, Oscar-winning actor!"). And the world fell to darkness. (insert dramatic pause) But he has returned." There you go. Even if it is a direct sequel, just don't say it. Again, things really are that simple. Just let everyone know what kind of story to expect. Couple that with some bad-ass-looking imagery and maybe one iconic shot of Superman, and you can recap as much backstory as you want before the film begins, the audience will arrive in droves, and they will be expecting it.
So we know the movie will do well at the box office, and all that was without even making blind assumptions about what we'll actually see in the movie AND without discussing how Singer can do no wrong. That's all just based on facts we've been given and marketing knowledge.