I use religion fairly superficially in my B/X games. I use the typical vast pantheon that I assume is pretty common in most D&D campaigns.
One thing I do find interesting is considering what a world would be like where not only is the existence of multiple deities a given but also where they take an active enough roll that their clerics have supernatural powers.
I take the easiest way out in my campaigns and just treat them as any other commodity/service in the game world. You need a sword made - you go to the weaponsmith. You need a curse removed - you go to the priest. They also become a great source for adventure hooks.
Three things that I think would happen in a D&D "Deities are Real" world (not saying they/him/her aren't in our real world):
1. The already mentioned commoditization of religious services.
2. There would not be any Atheists.
3. The fear of death would be largely removed. The great unknown would be answered. Dead adventurers have been brought back to life and are able to tell the tale. Heck, if you have a high enough level cleric or magic-user you could magically check out the afterlife ahead of time.