The gambiteer does not care about material. He only needs material for giving a checkmate -- which usually takes two or three pieces. The only fear is to be driven into endgame which would be lost by default.
To have less material usually means having more space. Naturally, when a pawn is sacrificed, its square is then free for a piece. Space is more concrete than material. It is relatively easy to count how many moves a piece has; of course, assessing the quality of those moves is another thing. Counting the number of pieces and pawns sounds easy, too, but the question is when the "predefined" values (pawn 1, knight 3 etc.) can be used. For example, quite a few DDG games could have been played without White's rook on a1. In other words, in those games the (effective) value of that rook was almost 0.
Even more important is increased mobility. For example, in the DDG Accepted 1. e4 e6 2. d4 d5 3. c4 dxe4, the e-pawn is sacrificed, whereupon the bishop and queen can come to c2 and d3, to create mating threats to h7. Similarly, without the f-pawn, the rook would stand on a half-open file, ready to join the attack. And, of course, not only stand, but also move quickly in the middle of the battle.
If an unambiguous value could be attached to each piece, sacrifices could even be given good reasons. For example, a sacrificed pawn might increase the value of the queen, bishop and knight, so that the total value of pieces would clearly be higher after the sacrifice.
Initiative is the power to create threats. The player with the initiative is usually also attacking. According to Steinitz, the player not only has the right, but the obligation to attack when his opponent has some weakness. In the DDG, Black's typical weaknesses are squares f6, f7, and h7. Is this a contradiction: Black has done nothing to weaken his position, yet he has critical squares? The amazing fact is that White can create threats to those squares, which makes them weak. This should be self-evident, yet most books guide that the player must have made a mistake to get a weakness.
The player with the initiative controls the course of the game. Keeping the initiative thus requires creativity, whereas the opponent only has to react to threats. By everyone's experience, attacking is easier than defending, and, what is more, errors in defending tend to be more fatal, typically leading to a checkmate or great loss of material. Errors in attacking usually only lose the initiative -- which, or course, in the long term may lose the game.
The defender has less alternatives in choosing his plan. One might think that his job would therefore be easier, but it seems that choosing the right plan is much more difficult to the defender. There is often only one way to survive, whereas there may be several ways for the attacker to win. Many brilliant attacking games have been "busted" afterwards: the defender could have saved the game with some imaginative move. In theory, every attack is over-optimistic; in practice, attacks are most likely to decide the game. Being active means being willing to win -- which is enough most of the time.
The game of chess is about even in the start position: White only has the advantage of one move. By sacrificing a pawn, White breaks the equilibrium, and most "rules" and guidelines can be thrown away. Suddenly it is Black who has advantage, namely material. But, at the same time, White gets more space. Using the space, White can gain other advantages. By moving pieces quickly to new squares, manoeuvreing, White can create threats. This is called time advantage.
Sun Tzu wrote in The Art of War: "A speedy victory is the main object in war." A short game is easy to understand, its ideas are straightforward. By following one or only a few clear motives, the game is instructive, and errors are obvious because they tend to be decisive. A short game is often won by the gambiteer, which makes it enjoyable: 5 games of 20 moves are usually more satisfying than 1 game of 100 moves. The game is easy to remember, and even play in the head, without a board.
"Quick slashing attacks which often win in 20 moves", advertised Tim Sawyer his book Alapin French, Tactics for White (1995). The DDG usually takes 24 moves. Lately I have tried to prove that the position after 1. e4 e6 2. d4 d5 is actually a chess problem: White mates in 22! (Hint: the first move is 3. c4 :-)
At first, it might look like in e-mail games, time and development are not so important because players have enough time to think. However, the player with more space has more potential moves, and his opponent's moves can be analysed more thoroughly. Besides, the player with less space cannot easily guess his opponent's plan, due to a great number of possible plans. The more you have space, the less you have to analyse.
In summary, we can define two most important strategical elements as follows: space is ability to move pieces to new squares, time is ability to move pieces quickly to new (and hopefully better) squares. And in a nutshell: less material, more space, increased mobility.
Example games are from the first DDG e-mail tournament, started in October 1996.
Jyrki Heikkinen - Stewart Sutton. How to eat space.
Thomas Erben - Jyrki Heikkinen. My longest game and only loss in the tournament.
For more information, see
http://www.funet.fi/pub/doc/games/chess/ddg/The second tournament will start in October 1997.