Too often, when they've got a team down, they don't put them away.
Two quick examples from last year:
1. Against UCF, BYU was up 24-10 midway through the 3rd quarter and ended up losing in OT.
2. Against Nevada, BYU was up 28-13 at halftime, allowed 4 unanswered TDs, and ended up losing 42-35.
I know these were the first two games after Taysom's injury. With Stewart thrown into the fire, coupled with GFGH, there were a lot of really short offensive drives for BYU. I'm sure the defense was on the field a lot, and probably got tired as the games wore on.
However, these are just a couple of examples from last year. This seems to be a trend I've seen for as long as I've been a BYU fan. I'm sure part of it has been not wanting to run up the score, but too often games that should be blowout victories, or at least no-doubt wins, have become nail-biters at the end, with some (like the two above) ending up as losses.
I'd just like to see BYU have more of a killer instinct. When you're up, put the opponent away and leave no doubt!