Why is f4 considered as bad in the opening? I really like to play openings that contain f4 (f5 as black) for a number of reasons.


It cannot be challenged with d pawn, which is initially protected by the queen.
When the king knight is developed, the f pawn is no longer blocked.
Unlike c pawn, it can be easily supported by castling kingside up to the board, and helps the development of the rook if exchanged.
After king's pishop fianchetto, the diagonal is open and the bishop is very strong there.
Even if exchanged early, after castling kingside, the king rook has a semi-open file, which makes it stronger.
Enables Ng5 move, which can be nasty sometimes.


But usually this move is considered bad, or dubious at the best by grandmasters and engines. What is the reason?



      [FEN ""]
      [StartPly "3"]

      1. e4 e5 2. d4 exd4 3. f4 Nc6 4. Nf3 Bc5 5. Bd3 Nf6 6. e5 Nd5 7. O-O O-O 8.
Kh1





      [FEN ""]
      [StartPly "3"]

      1. e4 e5 2. d4 d6 3. f4 exf4 4. Bxf4 Nf6 5. Nc3 Nc6 6. Nf3 Bg4 7. Be2
It is not really possible to answer this generally as chess is more complicated and it really depends on what black is playing whether an early f4 is good or not.

I guess, what you actually like is the combination of e4 and f4. There are many examples where f4 is perfectly playable, such as in some Sicilians, the French defence and other semi-open games. 

In open games (1. e4 e5) playing an early f4 tends to be somewhat dubious at the highest level, but is of course perfectly playable at the club level.

If you want a system opening with f4, why don't you play the King's Indian Attack, where "f4" is often ok whatever black does?