Originally Posted by MatthewSchwartz

You're asking too much if you expect to find it all on one site, but there are a myriad of sites that provide what you want.

Forums and discussion
Piano Society Forum :: Index (specifically intended for pianists)
http://www.youngcomposers.com/forum/ (specifically intended for composers)
Classical Music Forums - Talk Classical (for general enthusiasts)

Unfortunately, all three of these forums are pretty damn slow. Not a lot of traffic, so don't expect "instant-messenger-like" response time. But if you use all three, that should keep you company.