General opening principles

We can now add three more points to our rules for the opening. We must think about what play will be like in the middlegame. Masters do not simply bring their pieces into play and then start to wonder what they intend to do with the said pieces. After a few moves they will be already working out a plan for the approaching middlegame. If, for example, you want to mount a queenside attack in the middlegame, you should be taking this into account as you develop the rest of your pieces and make your pawn moves. 
A second important consideration is the establishing of the pawn structure. Establishing a sound pawn structure is very important. The future course of the game often depends on the pawn structure. You should avoid unnecessary weaknesses (e.g. doubled pawns or backward pawns). However, every single situation needs to be evaluated in specific terms: perhaps you will obtain sufficient compensation for the weaknesses (open files, a lead in development, a strong point, etc.).
The players (especially those with the white pieces) should be trying to seize the initiative. Having the initiative is an advantage!
It is often possible to sacrifice one or sometimes even two pawns for the initiative.

White's opening goal can be described as follows: in the shortest possible time he must bring into play the maximum number of pieces, seize the center, establish a healthy pawn structure, provoke weaknesses in his opponent's camp and take over the initiative.
Black's opening goal is: develop his pieces rapidly, fend off threats from his opponent, without weakening his own pawn structure, keep a firm eye on the central squares and fight for the initiative.
If your opponent makes a mistake in the opening, you must exploit it with the greatest possible energy.

You could even reverse the order of presentation of your point #1 and point #2, so that Pawn Structure became the first consideration rather than the second.

After all, your middle-game play will be determined largely by the Pawn structure. I'll give two examples, with all the pieces removed so that only the Pawn Structure is visible:

White has a half-open c-file, and more space on the Queen's side. His most natural plan involves dominating the file and advancing his Queen's side Pawns to open lines and create weaknesses.

Black has a half-open e-file and more space on the King's side. His most natural plan involves pressure on the e-file, plus a Knight outpost on e4, and if possible a King's side attack.

White has a half-open d-file and a considerable advantage in controlled space in the center and on the King's side. This should also give him a development advantage. White will be attacking in the center or on the King's side, since his advantage lies in those places.

Black has a half-open c-file and has two center Pawns (at d6 and e6) against one (at e4). Black can throw his Queen's side Pawns forward to engage White's King position, while putting pressure down the c-file with Queen or Rook. A sudden counter-blow in the center is also possible sometimes, where Black has a local preponderance of two Pawns against one.

The Pawn structure determines the middle-game planning... so perhaps it deserves first mention?

@2

"The Pawn structure determines the middle-game planning... so perhaps it deserves first mention?"
++ No. Modern grandmaster games happily accept a poor pawn structure

1 e4 c6 2 d4 d5 3 Nc3 dxe4 4 Nxe4 Nf6 5 Nxf6+ exf6 with doubled pawns
1 e4 c5 2 Nf3 Nc6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 e5 with a backward pawn at d6
1 e4 c5 2 Nf3 d6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 a6 6 Be3 e5 with a backward pawn at d6

You have totally missed the point.

Your first example: the doubled f-Pawns and half-open files DETERMINE THE MIDDLE GAME PLANNING.

Your second example: the backward d-Pawn and the half-open files DETERMINE THE MIDDLE GAME PLANNING.

Your third example: the same.

Where did I say that the quest for a perfect Pawn structure determines the middle game planning? I was referring to the actual position on the board... with its plusses, its minusses and its special characteristics.

@4

If you put it that way, I agree.
However, in post @1:

"Establishing a sound pawn structure is very important."
"You should avoid unnecessary weaknesses (e.g. doubled pawns or backward pawns)."
"establish a healthy pawn structure"
"without weakening his own pawn structure"
Imply that the quest for a perfect pawn structure determines the middle game planning.

Former USSR champion GM Iossif Dorfman lists in The Method in Chess four priorities in descending order:

1. King safety
2. Material
3. Position after a queen exchange
4. Pawn structure

Thank you.

Cease fire, men!

Just pointing out that you quoted me, instead of him.

@6

No hostility intended.
You endorsed @1 with "it deserves first mention".

It is an interesting question.
1 e4 c6 2 d4 d5 3 Nc3 dxe4 4 Nxe4, and now 4...Bf5 or 4...Nd7 do not spoil the pawn structure, but nowadays 4...Nf6 5 Nxf6+ exf6 is often preferred, accepting the doubled pawn.

Likewise 1 e4 e5 2 Nf3 Nf6 3 Nxe5 d6 4 Nf3 Nxe4 and now 5 d4 or 5 d3 do not spoil the pawn structure, but nowadays 5 Nc3 Nxc3 6 dxc3 is often preferred, accepting the doubled pawn.

Also 1 e4 c5 2 Nf3 Nc6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 and now 5...d6, 5...e6, 5...g6 do not spoil the pawn structure, but nowadays 5...e5 is often preferred, accepting the backward pawn.

Also 1 e4 c5 2 Nf3 d6 3 d4 cxd4 4 Nxd4 Nf6 5 Nc3 a6 5 Be3 and now 5...e6, 5...Nc6, 5...g6 do not spoil the pawn structure, but nowadays 5...e5 is often preferred, accepting the backward pawn.

It seems that chasing the knight (Ne4 or Nd4) from the center is more important than the doubled or backward pawn.

This is where a "History of the State of the Art" comes in useful... Tarrasch came before Alekhine.

Tarrasch (around the 1890s to early 1900s) formalized the static elements of the game, such as space control and pawn structure. His influence has been permeating the game of chess for over 125 years.

Dynamism was only introduced a generation later (in the 1920s), by Alekhine. The worth of his ideas took longer to be recognized (partly because Alekhine was such a jerk)... and Dynamism is also harder to formalize than subjects like Space Control and Pawn Structure.

So it's not surprising to see that dynamic plans (such as challenging White's centralized pieces) are still slowly reclaiming theoretical ground from the more static formal elements.

In fact, one would normally expect an over-reaction from Dynamism, driving theory past the equilibrium balance point until it is Dynamism, not the static elements, that is over-valued.

@8

The times of Anderssen and Morphy were full of dynamism, giving up pawns for activity,
e.g. 1 e4 e5 2 f4 or 1 e4 e5 2 Nf3 Nc6 3 Bc4 Bc5 4 b4.
The great champion of static elements was Steinitz, who went to great pain to win and hold a pawn, e.g. 1 e4 e5 2 Nf3 Nc6 3 d4 exd4 4 Nxd4 Qh4.
Tarrasch was a champion of dynamism: he voluntarily accepted isolated queen's pawns for active piece play, e.g. 1 d4 d5 2 c4 e6 3 Nc3 c5
Karpov was a static player, exploiting minor weaknesses.
Kasparov was a dynamic player, accepting backward pawns against Karpov.

I see variations where one side voluntarily accepts a weak pawn as gambits:
instead of giving up a pawn accepting a weak pawn but not yet lost.

In chess, the term "Dynamism" doesn't just refer to an active or sacrificial style (although that is one element in the mix). It has a somewhat more specific meaning.

Anderssen's style was regressive rather than innovative. He played in the Neo-Romantic "attack at all costs!" style popular in the first half of the 1800s. The Dynamic style was a Hypermodern invention, usually credited to Breyer (1910s) but brought to a peak by Alekhine in the 1920s.

Check Gyula Breyer for the originator of this approach to chess strategy. Sadly, he died young.