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.
*high quality* downloads from an infinite classical catalogue
Classic Cat - the free classical music directory (more than 4,800 recordings by hundreds of different composers; all sorts of different instrumental ensembles)
Piano Society - Free Classical Recordings and Sheet Music (more than 2,800 recordings by over 130 different composers)
Peabody Institute - Ensembles: PCO Concert Recordings (many great recordings by the Peabody Concert Orchestra)
Peabody Institute - Ensembles: PSO Concert Recordings (many great recordings by the Peabody Symphony Orchestra)
Audio - Columbia University Orchestra (a small selection of recordings by the Columbia University Orchestra)
MIT Chamber Music Society live recordings: Catherine McCurry, Juliet Gopinath, Mea Cook, Sunny Wicks, Vincent C. K. Cheung and friends (chamber recordings by the MIT Chamber Music Society)
ISGM Music: The Music Library (chamber music from the Isabella Stewart Gardner Museum)
Washington Musica Viva: Recordings (many recordings by the Washington Musica Viva)
Classical | iCompositions - Music (many recordings by independent musicians)
All of the above sites are completely free and legal. Classic Cat encompasses all of the sites below it, except for iCompositions, but I listed these sites because they're generally the "cream of the crop" for directly getting music to download. The quality is never FLAC-like, but generally it's pretty good, especially the Peabody and Columbia recordings. None of these are commercial recordings.
If you want exceptionally high-quality/commercial recordings, you'll have to use torrents.
half-decent search facilities to search on composer, ensemble, instrumentalist etc
Piano Society - Free Classical Recordings and Sheet Music (oriented for piano works)
Main Page - Wikipedia, the free encyclopedia (not to be underestimated; you can also check the References and External Links)
decent and up to date reviews
allmusic (personally I haven't visited here much, but it looks decent)
Amazon.com: Index: Classical Music (Jeff Bezos won't let you down)
Well, this should be more than enough.