Below are all relevant links I have found on reddit for Diablo 3. I really recommend to check them out! And please comment if you have other links you want to recommend.

General

http://www.reddit.com/r/Diablo/

http://www.reddit.com/r/diablo3

http://www.reddit.com/r/Diablo3Strategy

Hardcore

http://www.reddit.com/r/d3hardcore

Übers

http://www.reddit.com/r/infernalmachine

PvP (Brawling)

http://www.reddit.com/r/Diablo3PvP

Gear Checks

http://www.reddit.com/r/D3GC

Trading

http://www.reddit.com/r/d3loot

http://www.reddit.com/r/d3t

Looking for groups/players

http://www.reddit.com/r/D3L4G

Funny

http://www.reddit.com/r/Diablofunny