I look at how many QBs throw for 3k, 4k / TD and INT numbers / Comp % / Sacks taken / OL Sacks given up...
Total carries and yardage totals for HBs
Catch / total yardage and YPC for WRs
Tackle / Sack / INT numbers
Total O and D / Passing and Rush numbers for both sides, etc...
With Injuries, I run a few tests at 50 / 60 / 75 and see what looks / feels the most realistic...
I also keep an eye on last year's IRL stats and general stat trends for comparison...