Actually, the result of a team’s first game can tell a lot. Over the past five seasons, a team that blew out its opponent by 15 or more points in week 1 averaged about 8 ½ wins over the other 15 games, while a team that got blown out by 15 or more points averaged 6.86 wins over the other 15 games. That’s a difference of 1.65 wins. In other words, on average, a team that blows out its opponent week 1 will go 9.5 and 6.5 on the season, either side of which often grants a playoff berth. On the other hand, a team that gets blown out averages worse than 7-9 for the season, a sub-.500 record that puts them nowhere near the playoffs (unless they were in the NFC south last year).
Interestingly enough, the difference in expectations for a season between a team that suffers a one or two-score loss and a team that suffers a 15+ point blowout loss is incredibly small. In fact, the average week 1 one or two-score loss team ends up winning 7.07 games out of the remaining 15, a measly .2 more games than their blowout counterparts. This even still rounds to the same 7-9 season.
Conversely, on average, a team that wins its week 1 game by one or two scores ends up winning 7.82 games out of its final 15. This rounds out to 9-7, a record that sometimes makes the playoffs.
What all this says is that despite the unusual circumstances that surround week 1 (few injuries yet, extra time to prepare, no tape for the season yet), it still acts as a surprisingly good indicator of the success a team will have during the season.