i am a french player
that's not the point
what i mean is why not play the slav if there is some alternative to not blocking in your qsb. why play semi slav. semi slav is inferior to slav. in the french there is no variation where you have control over whether your c8 bishop is locked in. but in the qgd, you do, and that's the slav.
Some good comments here, so I'll only give a glimpse into the Semi-Slav universe.
First of all, practically every line is completely nuts! Learn a branch or two well, and you will dominate.
1.d4 d5 2.c4 c6 3.Nf3 Nf6
[3...e6 4.Nc3 dxc4 5.a4 Bb4 6.e3 b5 7.Bd2 a5 8.axb5 Bxc3 9.Bxc3 cxb5 10.b3 Bb7 11.bxc4 b4 12.Bb2 Nf6 13.Bd3 Nbd7 14.0-0 0-0 is the starting position of the Noteboom Variation main line]
4.Nc3 e6
[4...dxc4 is the (Open) Slav, which itself can go in 100 directions]
Now White has a big choice:
5.Bg5
[5.e3 Nbd7 6.Bd3 dxc4 7.Bxc4 b5 8.Bd3 etc. is the Meran Variation]
[5.e3 Nbd7 6.Qc2 etc. is the Anti-Meran]
5...h6
[5...dxc4 6.e4 b5 7.e5 h6 8.Bh4 g5 9.Nxg5 hxg5 10.Bxg5 etc. is the Botvinnnik Variation]
6.Bh4
[6.Bxf6 Qxf6 etc. is the Moscow Variation]
6...dxc4 7.e4 g5 8.Bg3 etc. is the Anti-Moscow Gambit
Except for possibly the Moscow, all of these lines are insane!