user315: A polite duplicate asked last week by @57Jimmy mentions an incidental matter I find interesting:

But when all the pieces are out, well placed, and I have castled, I have no idea about what I should do next. It might sound silly, but I feel that everything would compromise the balance I have just created.

Here, @57Jimmy does not refer to a particular game of chess but speaks of the transition from the opening to the middlegame in chess generally. I, too, have sometimes felt the same way. Perhaps some chess masters, maybe including the late world champion Petrosian, have felt this way, too.
Admittedly, it is not easy to distill a focused question out of @57Jimmy's interesting observation but the observation is sufficiently interesting (at least to me) that I shall try.
Question: how does chess reward or punish players who, going from the opening into the middlegame, seek to maintain the balance to which @57Jimmy refers?
I will give my own answer below but am more interested in answers others might give.

user12873: From an objective/theoretical standpoint, a player is "allowed" to disrupt the balance if he already has some kind of advantage (material, more development, etc). Such a disruption allows him to transform his existing advantage into another type of advantage. For example, if you have a development advantage, it's often beneficial to aggressively break through in the centre as a means of creating an attack.
But if the game is equal, one does not have the means to disrupt the balance (again, purely from an objective standpoint), and maintaining the balance is recommended.
However, trying to play idly is not equivalent to maintaining the balance. Playing idly is punished by chess, unless one's position is so rock solid that such passive play is permitted. The term "balance" means both players' advantages and disadvantages cancel out nicely to yield an equal game. Carrying out a plan and even changing the structure of the game is recommended, since it doesn't necessarily try to unbalance the game.
TL;DR: Chess usually punishes players who passively try to keep the game from changing. But chess also punishes players who try to disrupt the balance without just cause (i.e., some advantage). Going with the natural flow of the game will change things, but also keep the game in balance.

user18428: Yes! While openings tend to be positional play for both sides such as following basics for eg developing pieces and controlling the centre, the middle game is tactically played with much more room for making detrimental errors though usually not with one move. Balance is not about how the pieces look on the 64 squares after the opening has ended but how each player commands it's army thereafter. Aggressive play on one side and safe play on the other can mean the difference between a match ending in the middle game or reaching the end.

user924: The first chapter of Axel Smith's Pump Up Your Rating is named No Pawn Lever, No Plan, and he hit the bull's eye with that title. Plans come out of pawn structure and are usually about implementing one of the possible pawn levers in the position (it's when you move a pawn to square where it can be captured by an opposing pawn; also called a pawn break).

But when all the pieces are out, well placed, and I have castled, I have no idea about what I should do next.

Says nothing about the pawns. Or about any of the opponent's pieces, for that matter.
That "balance" is a red herring. If you have no plan, then how do you know your pieces are well placed. First decide what you should be trying to achieve, then see where to place your pieces. Including in the early opening.
Most classical openings try to achieve the e4/d4 pawn center, so 1.e4 and 1.d4 are the most important first moves. After 1.e4 e5, the most important plans are about pawn breaks -- achieving a favourable d2-d4 (with or without playing c2-c3 first) or f2-f4. 
Often the pieces are moved out first because of all sorts of details, but the goal is always to attack pawn e5 with a white pawn.
Similarly 1.d4 d5 2.c4 has always been the most important reply to the Queen's Gambit (2.e4 is less good because of tactical reasons, it can't be easily recaptured, but the idea logical), while 1.d4 d5 openings without either have a stodgy reputation as it takes so long for the action to really start.
The beginner advice about developing your pieces, castling early etc is important, but they should have been told at the same time to try to achieve pawn breaks.

315: <p>A polite <a href="https://chess.stackexchange.com/q/23599/315">duplicate</a> asked last week by @57Jimmy mentions an incidental matter I find interesting:</p>

<blockquote>
  <p>But when all the pieces are out, well placed, and I have castled, I have no idea about what I should do next. It might sound silly, but I feel that everything would compromise the balance I have just created.</p>
</blockquote>

<p>Here, @57Jimmy does not refer to a particular game of chess but speaks of the transition from the opening to the middlegame in chess generally. I, too, have sometimes felt the same way. Perhaps some chess masters, maybe including the late world champion Petrosian, have felt this way, too.</p>

<p>Admittedly, it is not easy to distill a focused <em>question</em> out of @57Jimmy's interesting observation but the observation is sufficiently interesting (at least to me) that I shall try.</p>

<p>Question: how does chess reward or punish players who, going from the opening into the middlegame, seek to maintain the balance to which @57Jimmy refers?</p>

<p>I will give my own answer below but am more interested in answers others might give.</p>
 12873: <p>From an objective/theoretical standpoint, a player is "allowed" to disrupt the balance if he already has some kind of advantage (material, more development, etc). Such a disruption allows him to transform his existing advantage into another type of advantage. For example, if you have a development advantage, it's often beneficial to aggressively break through in the centre as a means of creating an attack.</p>

<p>But if the game is equal, one does not have the means to disrupt the balance (again, purely from an objective standpoint), and maintaining the balance is recommended.</p>

<p>However, trying to play idly is <em>not</em> equivalent to maintaining the balance. Playing idly is punished by chess, unless one's position is so rock solid that such passive play is permitted. The term "balance" means both players' advantages and disadvantages cancel out nicely to yield an equal game. Carrying out a plan and even changing the structure of the game is recommended, <em>since it doesn't necessarily try to unbalance the game</em>.</p>

<p><strong>TL;DR</strong>: Chess usually punishes players who passively try to keep the game from changing. But chess also punishes players who try to disrupt the balance without just cause (i.e., some advantage). Going with the natural flow of the game will change things, but also keep the game in balance.</p>
 18428: <p>Yes! While openings tend to be positional play for both sides such as following basics for eg developing pieces and controlling the centre, the middle game is tactically played with much more room for making detrimental errors though usually not with one move. Balance is not about how the pieces look on the 64 squares after the opening has ended but how each player commands it's army thereafter. Aggressive play on one side and safe play on the other can mean the difference between a match ending in the middle game or reaching the end.</p>
 924: <p>The first chapter of Axel Smith's <em>Pump Up Your Rating</em> is named <strong>No Pawn Lever, No Plan</strong>, and he hit the bull's eye with that title. Plans come out of pawn structure and are usually about implementing one of the possible pawn levers in the position (it's when you move a pawn to square where it can be captured by an opposing pawn; also called a pawn break).</p>

<blockquote>
  <p>But when all the pieces are out, well placed, and I have castled, I have no idea about what I should do next.</p>
</blockquote>

<p>Says nothing about the pawns. Or about any of the opponent's pieces, for that matter.</p>

<p>That "balance" is a red herring. If you have no plan, then how do you know your pieces are well placed. First decide what you should be trying to achieve, then see where to place your pieces. Including in the early opening.</p>

<p>Most classical openings try to achieve the e4/d4 pawn center, so 1.e4 and 1.d4 are the most important first moves. After 1.e4 e5, the most important plans are about pawn breaks -- achieving a favourable d2-d4 (with or without playing c2-c3 first) or f2-f4. 
Often the pieces are moved out first because of all sorts of details, but the goal is always to attack pawn e5 with a white pawn.</p>

<p>Similarly 1.d4 d5 2.c4 has always been the most important reply to the Queen's Gambit (2.e4 is less good because of tactical reasons, it can't be easily recaptured, but the <em>idea</em> logical), while 1.d4 d5 openings without either have a stodgy reputation as it takes so long for the action to really start.</p>

<p>The beginner advice about developing your pieces, castling early etc is important, but they should have been told at the same time to try to achieve pawn breaks.</p>
