This was not an atypical situation: usually a consensus of three to five RBs emerges as the elite, you-can-count-on-these-guys fantasy prospects, and almost always at least one of those guys is a total bust. Of course injuries happen: you always know injuries can derail a fantasy season, and RBs are especially prone to missing games, so elite RBs often disappoint because they miss games. But there are also busts that wildly disappoint even when they do play. Go to any year, and you'll likely find a consensus elite RB that struggled to put up meaningful fantasy points even when he played.
This year, there is probably a consensus top four, but I see another guy sneaking in there enough that I'll call it a consensus top five: Adrian Peterson, Chris Johnson, Maurice Jones-Drew, Ray Rice, and Frank Gore. If I had to guess based on past history, two or three of these players will actually meet or exceed expectations, one or more will miss 4+ games with injuries, and one of these players will be a complete and total disappointment even when he does play.
It's impossible to predict who could miss games with injury: that can happen to anybody, even if some players have a scarier injury history in the past. But it's also tremendously difficult to predict the play-but-suck bust: if it were easy, that player wouldn't have been considered an elite prospect to begin with. Last year I would have predicted Turner the bust and Forte a reliable guy whose team context would require him to score a lot of fantasy points, but I was wrong.
Maybe the high bust rate of elite RBs is an argument against spending big money/high picks on RBs, instead choosing the more reliable QBs or WRs for your greatest resources. Or maybe we can try to guess which RB could disappoint.
Adrian Peterson (three consecutive seasons between 1,609 and 1,885 yards from scrimmage), Chris Johnson (two straight between 1,488 and 2,509), and Maurice Jones-Drew (14+ offensive TDs three of four seasons) have put up solid numbers for multiple seasons. Peterson, Johnson, and Jones-Drew are what they are at this point: unless they get injured, they will be consistent fantasy producers who also have tremendously high ceilings. That leaves two guys as possible busts: Frank Gore and Ray Rice.
Frank Gore has four consecutive seasons with 1,400+ yards from scrimmage: he's a very consistent RB. However, he only has one season with 10+ touchdowns. I fully expect 1,400+ yards from scrimmage from Gore once again (and he also has a career high of 2,180, so massive potential is there). But if things go bad for the 49ers, it's entirely possible that Gore puts up those yards with something like 4-8 TDs, which would make him a disappointing fantasy player.
Ray Rice was spectacular last season, rushing for 1,339 yards and catching 78 passes for 702 yards. I don't see a ceiling for Rice. However, he also totaled eight offensive touchdowns last season (seven rushing), even though the Ravens had a total of 22 rushing TDs last season. That's easy math: though Rice was clearly the team's best RB, they still got (or gave) 15 rushing TDs to other players. They're willing to use Rice all sorts of ways, but give goal line touches to other players. Furthermore, 78 catches and 702 receiving yards is outstanding for a RB; it's entirely possible Rice drops to a more conventional total for a good pass catching RB (say, 50 catches for 400 yards). With the addition of Anquan Boldin, some of Rice's targets might move elsewhere. And he's only had one good fantasy season, potentially putting him the the Matt Forte one-great-season-that-looks-like-the-first-in-a-run-of-productive-seasons-but-is-actually-just-one-great-season territory.
Do I think Ray Rice or Frank Gore will be a bust this season? No, I don't (in fact I just took Gore in an auction draft). But then, most fantasy prognosticators don't expect any of the elite RBs to be a bust this season, yet it's quite probable one of them will. If you're trying to guess which elite RB to avoid, I wouldn't be scared of Peterson, Johnson, or Jones-Drew at all, but I'd be a little more leery of Rice or Gore.