user8209: Gambit openings are usually based on sacrificing pieces to have much better piece development. 
Some gambits, like Queen's gambit, or King's gambit, I can understand the idea. However, I cannot evaluate Danish gambit. White sacrifices two pawns, has two developed bishops, but Black seems to be able to cover up pretty fast.
[fen ""]
1. e4 e5 2. d4 exd4 3. c3 dxc3 4. Bc4 cxb2 5. Bxb2

Although it is obvious that Black has no piece development, how can White play to take advantage of it?

user9030: This is an example for how it can go wrong for Black side.
[Event "?"]
[ECO "B02"]
[White "Jacques Mieses"]
[Event "Monte Carlo"]
[Black "Frank James Marshall"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/ b - - 0 1"]

[Setup "1"]

1.e4 e5 2.d4 exd4 3.c3 dxc3 4.Bc4 cxb2 5.Bxb2 d6 6.Ne2 Nc6
7.O-O Be6 8.Bd5 Nf6 9.Qb3 Qc8 10.Nf4 Nd8 11.Bxf6 gxf6 12.Nh5
c6 13.Re1 Be7 14.Qf3 Rg8 15.Nxf6+ Bxf6 16.Qxf6 cxd5 17.exd5
Rg6 18.Qh8+ Kd7 19.Nc3 Bxd5 20.Qe8+ Kc7 21.Nxd5+ Kb8 22.Rac1
Nc6 23.Rxc6 1-0


8209: <p>Gambit openings are usually based on sacrificing pieces to have much better piece development. </p>

<p>Some gambits, like Queen's gambit, or King's gambit, I can understand the idea. However, I cannot evaluate Danish gambit. White sacrifices two pawns, has two developed bishops, but Black seems to be able to cover up pretty fast.</p>

<pre><code>[fen ""]
1. e4 e5 2. d4 exd4 3. c3 dxc3 4. Bc4 cxb2 5. Bxb2
</code></pre>

<p>Although it is obvious that Black has no piece development, how can White play to take advantage of it?</p>
 9030: <p>This is an example for how it can go wrong for Black side.</p>

<pre><code>[Event "?"]
[ECO "B02"]
[White "Jacques Mieses"]
[Event "Monte Carlo"]
[Black "Frank James Marshall"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/ b - - 0 1"]

[Setup "1"]

1.e4 e5 2.d4 exd4 3.c3 dxc3 4.Bc4 cxb2 5.Bxb2 d6 6.Ne2 Nc6
7.O-O Be6 8.Bd5 Nf6 9.Qb3 Qc8 10.Nf4 Nd8 11.Bxf6 gxf6 12.Nh5
c6 13.Re1 Be7 14.Qf3 Rg8 15.Nxf6+ Bxf6 16.Qxf6 cxd5 17.exd5
Rg6 18.Qh8+ Kd7 19.Nc3 Bxd5 20.Qe8+ Kc7 21.Nxd5+ Kb8 22.Rac1
Nc6 23.Rxc6 1-0
</code></pre>
