Tarrasch Defence 6.g3 Ideas for Black

I don't really understand what Black is trying to do when White plays g3. Usually without g3 Black can threaten the kingside to gain enough time to develop all of the pieces. But with the pawn on g3 this isn't possible anymore. It's also a lot harder to put pressure on d4 since Black's bishop almost never gets to c5. And to make things more confusing Black will sometimes play Re8 and Bf8(?) which seems like a horribly passive thing to do when you have an isolated pawn. All the While White seems able to develop the queen and rooks in a number of ways without weakening the position. What exactly is Black supposed to do here? 

Black can play c4 but I don't like the usual endgames much at all.

The modern way to play the position is with 9...c4, creating a mobile queenside majority. This radically changes the character of the game when compared to the other tarrasch lines; here black is the one with excellent endgame/late middlegame prospects due to his potential queenside passer, while white is the one trying to speed up the pace of the game with his more active pieces (in the normal tarrasch lines it's just the opposite!). An alternative is 9...cxd4, which leads to more typical play and has been used by kasparov in his youth, but is a bit under a cloud right now.

On general terms, you shouldn't thing that the fianchetto takes away all black attacking potential; for example your usual idea of a rook lift on the third rank is still there, a knight on e4a strong attacking piece as usual, and arranging Bh3 to exchange the fianchettoed bishop is often a good idea.

Yes I don't know very much about 9...c4 but one review about a recently released book on this opening led to some rather unfavourable conclusions on this move:

http://www.chessvibes.com/reviews/review-the-tarrasch-defence

My interest was in the positions reached in the book rather than the book itself. Those two endgames in particular don't look like something I'd ever want to play if I could help it. One features double-isolated pawns that are on the same color of your bishop and the other gets you stuck with an isolated e-pawn. I don't mind the isolani but this is just overdoing it!

I don't think the review is unfavourable. But anyway, you should remember that there is not a single defence in the whole game of chess where after 20 absolutely perfect moves by white black is comfortable. White will always have a more plesant position if he plays well, but black is still ok as long as he plays accurately. If you don't like this, you should give up chess rather than giving up the tarrasch :P

More importantly, that position will never realistically arise in your games; at our level we will meet less critical play by white, and the great thing about 9...c4 is that by creating long term endgame chances anything but very aggressive play by white will be in black's favour.

Well, I think it's time to invest in a book. 9. ... c4 looking at the Aagaard and Ntirlis has a great deal of computer analysis and personally I am way too lazy to look into it. However, I do have the book and maybe I will. From your diagram, 9. Bg5 cxd4  10. Nxd4 h6  11. Be3, Black can play ... Re8 followed by ... Bg4 (rather than ... Bf8). The idea is to encourage White to play h3 then Black plays ... Be6. If White plays Ng5 to take the Bishop, Black takes back with the f pawn but White's pressure on e6 is diminished since White cannot play Bh3. It's been well analyzed but I have found only 2300+ players really know how to play it well for White.