I became opposed to the death penalty upon reading a post that innocent people are convicted and executed.
It might be me sometime.
Religion. When I started here, I was a socially conservative Christian. Now I’m a devout atheist (and I can spell it too!) Before the dope, I didn’t know it was OK to be an atheist.
I came around on a lot of gay rights issues too, but that might be because of the religious conversion.
On abortion, I’ve seen that the left’s ideas of right and wrong actually are as crazy as I always thought they were.
On foreign policy, I’ve found that there really are people who still thing that socialism is a good thing that would bring about flowers and puppies if only everyone got on board. And now I know I can pretty much dismiss whatever the left says about foreign policy, especially military issues.
So there you go. A 180, a 100, and two 0s (or should I say 360s?)
US gun control. The train left the station long ago, and my former Euro-weenie ideas of banning guns etc. are ridiculously naive in context. (That said I still oppose widespread protection-oriented personal gun ownership and thank my stars I live in a [largely] non-gun culture.)
I’ve also become substantially more questioning of available data to support any given claim - and therefore now have fewer friends. 
I don’t think there’s any question that socialism would be a good thing if everyone got on board. The problem is not socialism itself, but the fact that getting everyone on board is impossible. It’s the ideal ideology for some hypothetical race of naturally hardworking, generous and conscientious people.
That is to say, it’s no good for humans.
Not sure what socialism has to do with foreign policy though.
No reversals, but I have come across a few posts that have given me new ways of looking at certain situations. I consider those the hidden gems of the board - I never know where I will find them, and I shut down the computer feeling that I am at least a little wiser. Keep up the good work, Dopers. 
Kind of, I got my ass handed to me in the Game Room once that made me heavily reconsider my ideals on used property. I still personally try not to get used versions of things that aren’t out of print, but I was really, really crazy (I got to the point of saying only poor people who can’t afford books that the original authors still get paid for should use libraries, though I did make exceptions on things like housing) on that issue until that thread.
This one, on review, isn’t really what the thread is about but since I typed it up anyway -
I didn’t really become an atheist until I was here. The way I’m phrasing it doesn’t really reveal the facts of me being “religious” (note the not-really quotes), but it’s a pretty weird story I don’t even have entirely straight myself. I was sort of always an atheist, I (completely honestly here), thought that being a “Christian” was more like being a Trekkie or Tolkien… ite.
Even when I was little I thought it was like a fandom, I was convinced that my church was either leading me on or REALLY into it, it didn’t help (hurt?) that my church was one of those soft-liberal “Jesus is what you feel in your heart” churches that didn’t really impress upon me the full gravity of “believing in Jesus.” I thought wearing a cross or Star of David was like wearing a football jersey or some trinket from your favorite fantasy series. It wasn’t really until I came here that I figured out that, yes, people actually believed that stuff and actually protested funerals and stuff (!) because of their view of the works and made me realize that it wasn’t some lousy book club, and that if I didn’t really believe the stuff I should just be out. That’s not the entire truth either, it’s not that I don’t think I didn’t have a twinge of belief before that, but it was certainly wearing off by the time I got here, and I wasn’t THAT naive I think I always knew in the back of my mind that it was impossible for the religion to be that ingrained if people didn’t truly believe it, but it never really clicked until I came here.
I also was one of those “every theory at least deserves to be looked at” people when it came to the paranormal. And while I still admit I’m not totally over it and have a kneejerk twinge of anger when someone dismisses (say) ghosts outright because “it should never even be on the table as an explanation” even when they don’t even try to offer an alternate one (and I’ve yet to dismiss an alternate explanation someone spent more than 5 minutes thinking about) I’ve definitely cooled down on it a lot. Honestly, it doesn’t get me much in discussions of whether they truly exist, so much as the wet blankets who jump on any thread with the word “ghost” or “psychic” in the title as shorthand for “mildly creepy or amusing things that you can’t explain” and proceed to yell at everyone in the thread “THERE ARE NO GHOSTS” ad nauseum as if we didn’t know that. Bonus points if after having the OP explain that he doesn’t really believe in ghosts and they’d appreciate if the offending poster would calm down they drop the “this board is about FIGHTING IGNORANCE” line afterwords, but that’s another rant/thread entirely.
I kind of like your church-as-book-club theory, though!
Well, that was productive.
Same here.
I can’t think of many specific examples, but I’ve been on the Dope for longer than I cared about “issues” so the SDMB community has definitely helped shape my opinions and even how I engage in critical thinking.
Somebody just changed my mind about whether Ray Guy should be in the Pro Football Hall of Fame.
The Great One changed my mind about the unpragmatic Kyoto Protocol.