With a character like Superman, I can see the need to do the origin, since you kind of need to explain how such a fantastic person with superpowers would end up on our world.
...at least for his first major motion picture. From then on, future films really should just get down to business, rather than telling the origin again and again ( a la THE AMAZING SPIDER-MAN).
With STAR TREK, the series began in media res. The premise is simple enough that an origin story wasn't needed--the Enterprise is on a mission of exploration. It's a ship with a long history, and multiple Captains/crews. We never saw the ship being launched, or the crew coming together. Every episode was complete and accessible to a first-time viewer.
And yet, each one of the spin-off shows began with an "origin"--showing the launch of a ship and/or the assembling of the cast. And it all became part of an interwoven, complex continuty.
I think part of the reason that the general public knows TOS the best is because it was the least "hardcore fans only" series, with self-contained episodes, and no real "continuity" to speak of that people needed to keep track of.
With STAR WARS, part of the charm was the fact that the audience was thrown into the deep end of a totally new Galaxy, with all sorts of alien cultures and technologies that would take multiple viewings to soak in. The story was structured so that the backstory was there to flesh the story out, but wasn't essential, and wasn't the focus. It was just backstory. And, again, I don't think anyone walked out of the theater confused by the backstory. The film was complete and accessible to a first-time viewer.
All of the key pieces are there--the Empire came to power, Vader killed Father Skywalker, and, as the film starts, Leia is trying to recruit the retired Kenobi into the Rebellion, while also bringing the stolen plans to the Rebels. Pretty simple.
Of course, now that things are so convoluted, SW has lost a good chunk of its universality--general audiences would need a scorecard to comprehend the dynamics of all six films, of which the original is now only a small part.
After seeing PHANTOM MENACE, my own mother (who serves me well as an excellent barometer of the general public's perception of this stuff) was confused by the whole "prequel" structure. When I asked her if she knew who Anakin would eventually become, she said, "Luke?".
This all also raises the question--will the new films continue in the vein of the originals (by tying into the already-complex continuty), or will they be more stand-alone, general-audience-friendly films?
Worst-case scenario (for me): they'll play like fanfic-wankery retreads in the style of PROMETHEUS and NuTREK.