One good general one is debate.org though the format of the discussions are really more like a formal debate than a discussion but they have debates in all the categories you are interested in. They do have a forum also, that is a little more informal, but I’m not that familiar with that part of the site.
dailykos.com is a good one for political and social issues, with a few other things tossed in.
religioustolerance.org is a great site for religious topics. I mostly use it for reference but they have discussions too.
Those are sites I am familiar with.
If you’re looking for one stop shopping, a site with discussions about just about everything, you might try
topix.com
That site was recommended as a good forum in an article I found but I don’t know anything about it.
You also might want to take a look at www.big-boards.com
That site lists hundreds of online forums with short descriptions of the forums, the number of members in the forum and the number of posts made. That may help you find something you like and the numbers are good indicators of how active the forum is.
Hope this helps.