While I have many thoughts on the matter, I know that there are greater football minds than mine here on this site, so I am writing to hear your answers to the question. Nevertheless, I would like to frame the question with two assertions, straw men admittedly, but the kind that can help us understand each other a bit.
Preseason games aren't meaningful
To claim that a preseason game has zero meaning overstates its unimportance. Had it zero meaning, it wouldn't be played.
Preseason games, even the first one, tell us all we need to know about the team
Obviously not, especially not the first one. There are so many factors in terms of playing time, sitting starters, game plan, competition, and chemistry, such that there must be a limited amount of information we may derive from a preseason game.
The actual answer is somewhere in between these two extremes.
I think we can all agree that W-L's importance is negligible. Stats tell even less of a story than they do during the year. Incumbent starters (whose position is not at risk in competition) may be rightfully more concerned about injury than giving it their all. So some of these standards have to be reevaluated if not thrown out the window. Still, we can evaluate players on technique, and on what they bring when they are in competition or under criticism. Is there more?
My question is really less "how meaningful is a preseason game?" and more "how is a preseason game meaningful?" I would love to hear what you look for in a preseason game, and how it changes as the preseason progresses.
One link on the subject I saw recently at a different blog http://phinphanatic.com/2011/08/14/how-important-are-preseason-games/