Before we come up with all sorts of theories of why the Republican field was so weak in the 2012 presidential cycle (as Paul Krugman does here or Fred Hiatt does here) or wonder how Mitt Romney would have done against better challengers (as Steve Benen does here), it's just worth remembering that the real GOP field this time was at least Romney, Pawlenty, Perry, and Barbour, and perhaps also several others, including Palin, Thune, Christie, and Daniels. That's the real field that we should consider when assessing what Romney beat. Most of the others who showed up for debates and even took votes in some primaries, such as Herman Cain and Newt Gingrich and Michele Bachmann, were just sideshows. Whether deliberately planned that way or not, they weren't actually running for the Republican nomination for president; they were seeking publicity for one reason or another. Or, in the case of Ron Paul, seeking to alter the debate within the party on various issues. Or maybe they were fully committed to running but just had no idea of how to do it or what it would take to win. The point is that Romney really had nothing to worry about from any of them.
Once can certainly make the case that the actual group who ran from president in 2009-2011 was relatively weak. I'd say it was similar to several previous groups: the Republican field in 2008, and Democratic fields in 2004, 1992, and perhaps 1988. None of those featured real first-tier heavyweights, and each -- including the GOP 2012 crew -- had a handful of plausible nominees, people who had conventional credentials and were within the mainstream of their parties. As far as why this cycle was similar to those, it's mostly supply, not demand. The only plausibly top-tier heavyweights out there really weren't, since neither Sarah Palin nor (gulp) Dan Quayle really qualify, and there's no one out there similar to Ronald Reagan 1980, someone who was a major party leader for years. As many have pointed out, the next tier down, the solid respectable sitting or recently retired Senators and governors, were wiped out to a large extent by the big Democratic landslides in 2006 and 2008. And so the pool of potential plausible nominees was relatively small, before even starting to worry about anything else at all.
The key point here is that Pawlenty at least, Barbour almost certainly, and at least a few of the others were defeated by Mitt Romney, even if those defeats didn't take place in Iowa, New Hampshire, or South Carolina. It's just become the case that Republicans winnow early, but that doesn't mean that the first ones out were actually the weakest candidates or had the smallest chance of winning. And if you stack the 2012 losers against other fields of losers, you're not going to find a huge difference on paper (where Rick Perry looks a lot more impressive, granted, than he turned out to be), and perhaps not even a big difference in fact (since Perry goes with John Glenn, Phil Gramm, John Connally, and other famous flops). It's rare to have a runner-up as strong as Hillary Clinton 2008 or Bob Dole 1988; there are a lot more like John Edwards 2004 or Mike Huckabee 2008 or Bill Bradley 2000 who aren't going to be more impressive than Perry and Pawlenty this time around.