Out of all the preseason football I think the first couple of games are the best. You usually have some on the bubble guys that are desperately trying to make the team and will be back at the local 7/11 if they don't.
Also most causal fans probably don't care about this but you do start to get a measure of some of the offensive and defensive schemes the team might try to use and how they want to use some of the new personnel this year.
I couldn't have said it better. Game three of the preseason is when you can get an idea of how good your team will be as the starters play into the 3rd qtr. for the most part. Game 4 is a total wankfest with every one trying to not get hurt.