I think it’s because it’s attached to the Straight Dope, for one. By and large the people around here are trying not to be stupid. That’s a cut above 90% of all the other boards out there.
Another factor has to be the sheer diversity. All these different people, all these different topics.
What do you think?
It’s the moderating. Boards with no or excessive mod-ing suck.
Good mods who don’t let the Nazi/OMGLIBERALAGENDA/OMGUBERCONSERVATIVE BS bother them at all.
An intolerant membership. Now, that doesn’t mean we hate people, but we don’t suffer fundies, morons, people who can’t type, conspiracy nutballs, etc… Consequently, they either don’t come in at all, or they post a bit and get annoyed when we laugh at them. Keeps out the riff raff.
There’s a strong core of posters in each forum. And I think you’ll find members generally care about each other outside the message boards. Witness the Dopefests and the occasional fundraising when a Doper is in need. This is not just some Internet thing for a lot of us.
If you want to see what we COULD be, just click Read/Post Messages at the end of any Yahoo News story and quake with fear.
I think it’s more along the lines of people at the Straight Dope having pretty high standards of proof. Granted, this ain’t ALWAYS the case - especially when it comes to subjective matters (which will be argued until eternity) - but essentially, any time someone claims to, oh, see Alien Shuttles landed on the Moon, a huge amount of incredulity is flung in their direction.
But anyway, you’re missing the REAL reason why the SDMB is so great… it’s because I post here. 
You know, there has been discussion in the past that we actually spoil each other here what with the level of intelligence of those who post, the need to refute bunk with actual facts and proof, and the all around use of excellent grammar.
I’ve been annoyed and sickened by nearly every other board I’ve looked at after becoming a member here, and though some might declare we have succumed to snobbery, that usually implies a certain amount of wealth, so I’ll just say we’re stuck up.
I’m a relative new-comer here, but I believe it’s such a nice place because of two big reasons. One, it’s basically founded on the principal of expanding knowledge (which you may have meant by saying it was attached to the SD, Tentacle Monster). Two, like Doomtrain said, the population is relatively intolerant of unpleasant types and the Mods are all reasonable people.
Also, I think the Pit probably plays a role in keeping the rest of the board civil. It provides a semi-regulated forum (no pun/double entendre intended) in which to vent anger at other Dopers…so threads outside of the Pit don’t get clotted up with flames.
Because there’s no l33t!
One simple rule: Don’t be a jerk. That, and we’ve got the moderators who will enforce it. While no doubt a board could and might exist where I could learn as much on as wide a variety of subjects as I can on this one, the fact that people here are basically polite are what drew me in and what keeps me here. I still remember being stunned when I first came here and realized that people were discussing abortion passionately yet politely.
The lack of leet-speak and the emphasis on being able to communicate well is nice, but the absence of jerks is what makes it possible for this place to be the community it is.
CJ
I think it’s a great community, but there really isn’t much diversity. Anyway, I like it mostly because of the members and staff.
All in all, I am impressed by the membership. In GQ right now is a fairly educated (and completely civil) discussion on How Molasses Kills. (We all know how quickly a hot topic like this can get out of hand.) A remarkable width and breath of experience out there.
The only places that come close are the Wikipedia and Tank Net.
It’s the creamy nugat center!
w00t!
I was thinking almost the same thing. This board is so great because $DOPER posts here. (Insert appropriate values for $DOPER.)
because it’s free!
oh, wait …
Not much diversity? We have:
• People from their mid-teens through their 60s
• Wealthy, poor, working, unemployed, students, retired people
• Every race, I think, under the sun
• Every religion and sect (though not enough Muslims—and, oddly, no Quakers!), plus atheists
• Far left, far right, everyone in-between
• About a dozen countries represented
• Straight gay, bi, trans and uninterested
• The good, the bad and the ugly
What other message board is that diverse?
I’d vote for the diversity, tolerance, and Mods who don’t take any shit.
This is key. It means you rarely have to suffer leetspeek, improper or nonexistent punctuation or horrible spelling, which goes a long way towards making reading threads a joy instead of a chore.
We’re also pretty fucking intelligent. There are exceptions, but generally speaking, we’re brilliant. Go us.
Not more than that? Off the top of my head, we have:
Canada
Saudi Arabia
USA
Australia
New Zealand
Germany
Great Britain
Sweden
Denmark
Finland
Norway
Japan
That’s a dozen right there. And surely there must be some French/Spanish/Italians/Russians, not to mention Asians and South Americans?
[Tacky Kumbaya-like feeling]
(Adapted from “We are the church”… I don’t know the author. )
The Dope is not a building.
The Dope is not a steeple.
The Dope is not a BBS.
The Dope is the people.
I am the Dope.
You are the Dope.
We are the Dope together.
[/Tacky Kumbaya-like feeling]
Gosh, there are so many reasons.
The emphasis on clear, intelligent, reasoned thought.
Understandable posts. Nothing worse than trying to parse someone’s half-assed paragraph because the concepts of grammar, spelling and punctuation are lost on the poster. From another board: “rofl lol theres never ne thng new” Who needs that?
The ability to actually complain about moderator actions. You wouldn’t believe how many boards have the “mods are gods” philosophy and don’t allow dissent among the ranks.
The ability to call out other posters. Those boards where posters are not allowed to disagree, be sarcastic, or otherwise do anything but pat the other posters on the back (unless the poster is disagreeing about something the board as a whole believes in) are frustrating. When someone is doing something stupid, you can’t tell them they’re doing something stupid. Here you can.
The humor. This is one of the wittiest groups of people I’ve ever met.
In other words, not only the info-centric “fighting ignorance” credo of the Straight Dope but also the tone of the place, as set by himself, Cecil Adams. Being abrupt and to-the-point and, where applicable, condescendingly insulting towards people promoting baseless assertions and/or whining without ground. We don’t roll over for fools just because they bleat and self-righteously appoint themselves the arbiters of moral/social/political propriety. We don’t incorporate the commonly-held attitude of “you can’t criticize So&So caustically even though most of us think that post was bullshit because you might hurt So&So’s feelings”. Yes, even the Pit plays a good, positive role in thinning the weeds around here. We not only have a board culture that values logical argumentation, defensible assertions, and a reasonable acknowledgement of other folks’ freedom to hold and voice other opinions in places where opinions hold sway, we also have a board culture in which the public disemboweling and roasting of the ignorant is a celebrated participatory and spectator sport. On other boards, disruptive people may eventually have their posts edited or deleted or find themselves banned, but it’s done in such a low-profile way that there are fewer obvious disincentives for people to pontificate vacuously, whine, or troll.
Second, good modding, as has been noted, but not merely good in the sense of rigorous — good also in the sense of (despite troll-assertions to the contrary) no Sacred Cow elderposters from whom all manners of posts are tolerated. Popular posters, long-term posters, even former moderators get their chops busted here when warranted. There may or may not be cliques and cohorts amonst the Teeming Millions within which there may be more leeway granted in debates and other conversations, but the mods as a team do a damn good job of being even-handed
both in terms of issue-positions and in terms of board personalities.