What is wrong with f2-f4 against the French Defense? [fen ""]
[Startply "3"]
[Title "2.f4 against the French Defense"]

1.e4 e6 2.f4


What is so obviously wrong with this approach? I ask because it is not even mentioned in the complete Black repertoire book French Defense: The Solid Rubinstein Variation by Hannes Langrock http://www.russell-enterprises.com/images/frenchrubinsteinexcerpt.pdf which includes second move alternatives to 2.d4 such as 1.e4 e6 2.b3 or 2.Qe2.
Grandmaster Igor Glek wrote a survey about 1. e4 e5 2. f4 in Secrets of Opening Surprises, volume 8 (2008).
Some quotes:

With 2. f4 we return to the nineteenth century, when modern chess understanding made its first steps. The basic idea is clear I suppose - after 2... d5to play 3. e5! - gaining some space in the centre. So in principle we see the same ideas as in the popular Advance Variation (2. d4 d5 3. e5), but White doesn't permit Black to create immediate counterplay against pawn d4.
...
My opponents usually played in the same way as they would against 2. d4 d5 3. e5 line - ... c7-c5, ... Nb8-c6, ... Ng8-e7(h6)-f5. My reaction was Nb1-a3-c2and then not d2-d4 immediately, but first Bf1-d3. Interestingly, we can already see this manoeuvre in McDonnell's and Labourdonnais' games!

That's exactly the plan Nakamura followed in the game mentioned in the comments.
[FEN &quot;&quot;]
[Event &quot;US Championship&quot;]
[Site &quot;Saint Louis USA&quot;]
[Date &quot;2012.05.19&quot;]
[Round &quot;11&quot;]
[Result &quot;1-0&quot;]
[White &quot;Hikaru Nakamura&quot;]
[Black &quot;Yasser Seirawan&quot;]
[PlyCount &quot;59&quot;]

1.e4 e6 2.f4 d5 3.e5 c5 4.Nf3 Nc6 5.c3 Nge7 6.Na3 Nf5 7.Nc2 h5
8.Bd3 g6 9.O-O Be7 10.Bxf5 gxf5 11.d4 h4 12.dxc5 Bxc5+ 13.Be3
Be7 14.h3 b6 15.Qe2 Nb8 16.Rfd1 Ba6 17.Qe1 Nd7 18.b4 Nf8 19.a4
Bc4 20.Ncd4 Qd7 21.b5 Ng6 22.Nc6 Kf8 23.Nd2 Bd3 24.c4 Kg7
25.cxd5 exd5 26.Nb1 Bc4 27.Qc3 Qe6 28.Nd2 Rhc8 29.Nd4 Qd7
30.e6 1-0

So the answer is: There's nothing wrong with 2. f4 against the French Defense.