The Detroit Lions faced the NFL's toughest schedule in 2012 based on combined wins and losses. Their schedule was littered with playoff teams, and it had a lot to do with their 4-12 record. That's not to say it's an excuse, but the Lions had a very difficult schedule last season.
If you base the Lions' 2013 schedule on results from last season, they once again have a difficult schedule. In fact, they have the second-most difficult strength of schedule for 2013, according to NFL.com. Lions opponents went 138-118-0 last season, which is good for a .539 winning percentage. Only the Carolina Panthers have a more difficult schedule (.543 winning percentage).
Of course, the caveat with any discussion like this is that teams that were good last year could be bad this year and vice versa. Just look at the Lions from 2011 to 2012. They went from 10-6 to 4-12, meaning they weren't really as tough of an opponent as people would have thought at this time a year ago.
While this is interesting to see, it's really not possible to determine how tough a team's schedule will be until the games actually begin. Even after free agency and the draft it's tough to predict how good or bad a team will be, so although the Lions' 2013 schedule appears tough, we could have a much different view of it by the end of the year.