"The first move in a game of Chess isn't even CLOSE to the most important
one in a typical game."

Obviously, additional explanation of my meaning is needed.

In terms of a chain of events leading to a final outcome ...
the first (a move, in the topic under discussion) is always 
the most important because it has a determinative effect 
upon not just itself (as the last move of the game does) 
but all (moves) that follow.  Even though the very first move
of the game (by white) is not the most exciting,
it (moreso than any other move) determines the course of the
game as defined by its unique move list.

In Chess, where a strict white-black turn order exists, 
all hypothetical talk of non-existent double-move options is
completely irrelevant.

"I also see no particular reason to think that a Bishop moving 7 squares
has equivalent value to taking 7 consecutive moves in a game of
checkers--but if it were true, that would seem to severely undermine your
theory that the first move in Chess is the most important one, since no
piece can 
move farther than 2 squares on the first turn."

Technically, you have one point that should be addressed.

No.  White cannot move any piece of unlimited range on the 
first move of the game.  However, by advancing an appropriate 
pawn on the first move, white can then move a queen or bishop 
diagonally on the second move of the game.  [Note:  I don't
recommend actually doing so.]

The important point is the equal burden of development by 
white and black does not diminish the significant, measurable
first-move-of-the-game advantage by white in Chess which 
undeniably exists and is all-but-proven statistically via a 
vast number of reasonably well played games.  After all, 
white has a head start toward this development.