A nice summary of the reasons via Black Belt Bayesian, one of the highly recommended blogs on the Accelerating Future domain. I especially liked this point:

Ideas have changed too little. In Star Trek’s society, as far as I know, there is no taboo of ours that has become universally accepted. Yes, the mores of Star Trek’s society are such that we consider them progressive, but progressives as little as 100 years ago would be shocked if they could see what sort of things we consider normal. It’d be unlikely if there were nothing in future customs to shock us. There don’t seem to be any genuinely new ideas on how to have society work, either. I’m thinking along the lines of prediction markets, or even just blogs. Like with many other points, I don’t blame the writers for this; it is in predicting the future of ideas that futurism runs into its hardest limits. But a future with no weird ideas is still deeply unrealistic, and that’s worth keeping in mind.

Stephen and I were just chatting about how any compelling discussion of the future has to get into the weird stuff sooner or later. And that's not just social conventions, where I would agree that Star Trek dropped the ball. Once in a while, you would see a truly mind-blowing concept -- an abandoned Dyson Sphere, an alien race that speaks only in literary metaphors, a species that grows its population by resurrecting the corpses of other abandoned life-forms -- but more often than not you would get a lot of tried and true (and generally quite entertaining) stuff about Klingons and diplomatic crises and, of course, a "form of energy never encountered before" which causes problems for 55 minutes, only to be rectified in the last 5 after a Level One Diagnostic inspires a truly ingenious solution, usually involving the Main Deflector Dish (and reversing the polarity of something.)

Another idea missing from Star Trek -- not, as the post points out, that the writers can be blamed for it -- is the idea of a technological singularity. The closest Trek ever came to that idea was the end the first Star Trek movie. And even in that setting, there was this attitude of "maybe for thee, but not for me." Relative to Speculist readers, the folks in Star Trek are relative luddites.

Remember when Q gave Riker all the powers of the Q continuum? Riker gave them back within the prescribed 60 minutes out of fear that he might "turn into something else." There was this concern that he was becoming arrogant -- he was doing things like addressing the captain by his first name!

I suspect most of us, given a similar offer, would handle it differently.

For example, how about keep the powers and don't act like a total schmuck? Think of all the good he could have done for humanity if he kept them only for a week. Or if that's too risky, think of all the unbelievably hot sex he could have had. (Just to put it in Riker-friendly terms.) In one episode, I remember Riker confessing to Picard that he didn't ever intend to die -- wow, those Q powers might have really helped with that one, buddy.

Don't be a schmuck, man

Actually, that would have been a fun device, if it had ever occurred to the writers. A couple of seasons later, have Riker get killed and then suddenly pop back to life -- whereupon he confesses that he did, indeed, keep a little Q Juice for himself when he supposedly renounced those powers. He just set it up so that he's immortal and unkillable. That would be an interesting quandary -- what do you do on a starship where you have several hundred normal, vulnerable crew members and one guy who cannot be killed, no matter what? I guess he would become a sort of one-man away team.

Of course, Data could also have been that indestructible crew member. Have him run a full backup before every away mission. If things go well, they go well. But if Data gets blown up, well we just replicate a new model and upload the backup. Uploading (even for the freaking android character), life extension, cryonics -- these ideas made scant appearances in Star Trek, and usually only for the purposes of poo-pooing them. Granted, these ideas are hard to package into entertainment products. The Matrix gives us a post-singularity world where conflicts between human and artificial intelligence are handled by elaborate martial arts fights and putting together (and unleashing!) massive arsenals of personal ordnance. A more "realistic" handling of some of the same issues can be found in a movie like Vanilla Sky -- but I'll take Star Trek or the Matrix over that, any time.

UPDATE: An alert reader reminds me that it was not an episode of TNG in which Riker declared his intention to live forever, but rather the end of the movie Star Trek: Generations. This reader writes:

Riker confessed to Picard that he intended to live forever at the end of Star Trek: Generations, not in a series episode. It went something like this:

Picard: After all, we're only mortal.
Riker (grinning): Speak for yourself, sir. I intend to live forever.

(just paraphrasing; if you really want I can pull out the DVD and quote word-for-word!).

Don't trouble yourself, friend. I think we got the gist. So perhaps Riker only came to his desire for immortality long after turning down the Q powers. In which case I can only remind the sometimes-bearded commander that in this life, timing is -- if not everything -- pretty darned important.

Schmuck.