morphyvsfischer: Kasparov apparantely didn't trust his pet 4 Nf3 here.4...0-0 5 a3 Bxc3+ 6 Qxc3 b6 seems more Anand's style
4...c5 5 dxc5 and now instead of letting White get a small advantage in space with ...0-0 6 a3 Bxc5 7 Nf3 Nc6 8 Bg5, an exciting game could go 5...Na6!? 6 a3 Bxc3+ 7 Qxc3 Nxc5 with a fascinating position.
5 ...exd5 Bg5 and white has an edge.
6 Nf3 Qf5! (not allowing the white queen to guard e4) 7 Qxf5 exf5 8 Bd2 c6 9 e3 Nbd7 10 Bd3 Nb6 11 Ne2 Bd6 12 0-0 Ne4 13 Ba5 0-0 14 Rfc1 Rc8 and Black has a very good position, with slightly better central control.
7 a3 and now either ...Bxc3+ (avoids loss of time) 8 bxc3 Nc6 10 c4 Qd6 11 Bb2 cxd4 12 exd4 b6 13 Bd3 Bb7 and black has good play against the white pawns, or ...cxd4 8 axb4 dxc3 9 bxc3 b5! (blocking White's development) 10 Nf3 0-0 11 c4 bxc4 12 Bxc4 Qe4 with the equal game refutes it.
10 Bc3 leaves Black with the initiative: ...e5 11 Nf3 Bg4 12 Be2 0-0 13 Qb3 Qxb3 14 axb3 Rfe8 15 Bb5 Nd7 16 Bxc6 bxc6 17 Ra5 f6 and the game is equal.
Black should avoid an immediate 17...Bc6: 18 Qxd8+ Rxd8 19 Rxd8+ Kc7 20 Rhd1 Nd5 21 R8xd5 Bxd5 22 Nxd5+ exd5 23 Bf3 and white will win the pawn on d5, and the black queen has little to attack.
These side variations still don't explain why Kasparov played so timidly.