Everyone loves The Bus, and to most, his career numbers justify him as a bona fide Hall of Fame candidate. He is 6th in total career yards, 5th in total career rushes, 10th in rushing TDs, and led on offense for a team that dominated a division for most of time there, winning numerous division titles, playoff games, a few conference championships, and of course, a Super Bowl ring. Some same that this last fact alone means that he was robbed when Curtis Martin was installed in the Hall ahead of him, since other than the ring, very little separates Bettis and Martin in terms of the statistics listed above. The fact is, everyone else with numbers like those is in the Hall. However, Bettis has one big knock against him that is brought up time and again when he is compared to what should otherwise be his peer group of HOF backs: his career yards per carry. The Bus is the only back who is in the top 10 for career yards and for carries who has a sub-4.0 yards/carry for his career (Bettis is at 3.93). In fact, Bettis isn't even in the top 200 running backs in terms of yards/carry. There is the potential for a lot of debate as to why this may be, but without a doubt, it is one of the primary reasons that he was not a first (or second) ballot Hall of Famer.
The question really should be: is this "deficiency" really that relevant to his actual impact as a player and/or are there are any additional statistical or non-statistical factors unique to Bettis that perhaps make up for this perceived weakness relative to his peer group. I think there are last two qualities that Bettis has that more than make up for this deficiency (and maybe even partially explain it) and merit his installation among his peers.
The first is the often discussed, but not always deeply analyzed, aspect of Bettis being perhaps the best "big back" ever. The fact that a "big back" achieves comparable career numbers to other more typical running backs is not really a basis in and of itself to say that Bettis belongs in the HOF, just because he happens to have a different body type. If that were true then the best "tall QB" or the best "right-handed guard" should also get special consideration if their numbers are close to their more generic statistical peers.
I think the question really is: whether being a "big back" made Bettis more valuable to his team beyond simply allowing him to rack up yards and carries. There's really no way to generate any data on this subject, but I think the answer is probably yes. Part of a running back's job is to advance the football as much as possible, and in that vein it could be argued is that all that matters is how many yards he gets per try. It is this thinking that makes people question Bettis's value relative to guys who averaged more than 4.0 yards/carry and had similar career number. However, in today's statistically-obsessed fantasy football world, it is easily forgotten that running backs have another job as well, particularly big, between-the-tackles backs: to beat up and wear down the opposing defense over the course of the game. I think anyone who watched Bettis literally run over Brian Urlacher in the pivotal Week 14 win over Chicago in 2005 on the road to the Steelers Super Bowl victory that year understands that Bettis is certainly one of the best ever at punishing the opposition, even compared to some of his statistical peers, such as greats like Dickerson or a Sanders.
Because it is hard to measure the damage a bruising runner can do the morale, physical integrity, and focus of a defense over the course of a game, it is understandable that people tend to focus almost exclusively on a running back's ability to move the football. Yards are certainly important -- and Bettis has plenty -- but to ignore the running back's impact on the defense is to ignore an equally essential and valuable aspect of a feature running back's job -- to allow the offense to impose its will on the defense and set up everything else that the offense wants to do. It can't really be measured, but you could certainly make the argument that you might even be willing to exchange a few yards gained for dishing out a little additional punishment to opposed defenses. Bettis averaged about 18 carries per game in his career, gaining about 71 yards per game on average. This places him at a respectable 34 (right next to the likes of Ray Rice and Gale Sayers). But one could certainly argue that because of Bettis's style, the punishment he dished out relative to other great backs (particularly elusive style backs) might not always have been worth gaining 5-10 additional yards per game. In fact, it is this head-first style which in a liklihood contributed to his relatively unremarkable yards/carry. Such a debate can't really be resolved conclusively, but I think there's no doubt that punishing backs bring additional, non-trivial intangible value to their teams, and that Bettis certainly did. Just because you can't measure it doesn't mean it shouldn't count.
The second thing that people (especially non-Steelers fans) don't always appreciate about Bettis is his ball protection. With one very obvious exception, Bettis was always regarded as a sure-handed back who rarely fumbled, and never at key moments. But how does he really stack up when compared to his peer group? The answer is: extremely well. Bettis fumbled only 41 times in his career, meaning he fumbled on only 1.18% of his carries or 0.3 fumbles for every 100 yards rushed. When you compare these fumble numbers to backs that who are in the top 10 in career carries and yards, most of whom are already in the Hall, Bettis comes out ahead of almost everyone. In fact, of current HOFers in the top 10 in these categories, only Curtis Martin is ahead of Bettis with a fumble % of .82% and .21 fumbles/100 yards. Among current or recently retired top backs, only LT and Steven Jackson put up better fumble numbers, and then, only slightly better. Bettis is a statisically better ball protector Marshall Faulk, Barry Sanders, Emmitt Smith, Shaun Alexander, Edgerrin James, Adrian Peterson, Marcus Allen, Sweetness, Jim Brown, Franco Harris, and Tony Dorsett. This is should put rest rest whether Bettis has earned his vaunted status as a sure-handed "closer." (See stats below)
Between these two factors, Bettis, despite his subpar yards/carry, more than makes up for any shortcomings relative to his peers among the great backs of all time. While certainly no one is arguing is he is best running back ever, there is a strong argument that he is a top-10 back, and there is no doubt at all he deserves a place in the Hall.