Nowork pointed me to this article over at 29inches, which I thought was pretty interesting, and I (as the personal owner at various times of both 4, 5, and 7" travel 29ers) tend to generally agree with the conclusion - that the 4" 29er is the "all around XC" bike for most folks.
I thought I'd offer a bit of the designers perspective on why I think that longer travel 29ers will remain a niche product. There are a bunch, really, from 2 angles - fitting the rider, and fitting the rear wheel.
The rider is the easiest part to start with, though both problems interact to a significant extent. When we add travel to a 29" full suspension bike, we start running into some standover problems.
Now remember, I'm on the record as saying I don't think standover is all that important, as long as you can comfortably get on and off the bike. And it's easy to build a hardtail 29er with standover well under 28" or so (without even resorting to any weird trickery), which means that most non-tiny folks with shoes on can ride them pretty comfortably. The dualy is a different story than the hardtail, of course - we'll be using some sag in both the front and rear suspensions when the rider is onboard, so to get the BB height where I want it, the whole bike gets taller. A 3" dualy, all things being equal, is going to have about 3/4" less standover than a suspension corrected hardtail. And every inch of travel we add will basically add another inch to the standover height. So as you go up, you eliminate folks who can comfortably mount/dismount.
Now consider, also, that as we add travel (at least on most single pivot frames) we've got to make room for the rear wheel to track up (as the suspension compresses) and towards the seat tube (this problem is mitigated to a very small extent if you've got a slightly rearward wheel path, but not that much - the seat tube is still in the 73ish degree range, so you'd need a *really* rearward axle path to keep the tire away from it entirely). And of course, you've got to make sure you can run a front derailleur, and all the associated cable clamps and stops and such need to clear the rear tire as well.
That wouldn't be a problem, except that the rear ends of 29ers are already on the long side (an average 29er hardtail is probably in the 445-450mm range for effective chainstay length). As we add travel, we have to either move the seat tube out of the way, or lengthen the stays to keep everything from smashing together - so looking at some of the popular bikes out there, chainstay lengths are in the 450-470mm range, with a few even longer. Those are mostly 4" bikes. As we add travel, we have to do more and more futzing to keep everything working right, or add more length to the chainstays.
Now there's nothing wrong with long chainstays for many people. Lots of people whine and moan about not being able to "manual" easily, for example, because 29ers have chainstays that are too long. Without getting too far into this debate (it would end up being a rant about how chainstay length should be custom for every rider and all sorts of lengths can be "right" depending on terrain and preference), while most of these people don't even know what an actual manual is (let alone actually being able to ride one), they do have a point, to an extent. If you're 5'10" tall and have 470mm chainstays, getting the front end up is, all things being equal, harder than if you had 445mm ones. The more travel, the harder it becomes to make chainstays short, meaning that you end up with a bike that works well for certain riders but really poorly for others. Not so good.
But at 4" travel, most of this can be pretty easily overcome. A 4" travel 29er (which is what I ride on many days) can handle most reasonable XC riding, and even a bit of chairlift-served silliness as long as you don't go too crazy. I think that the author is right - we'll see 4" settle in as sort of the standard for 29ers. What surprises me is that 3" bikes are virtually nonexistent - I can make a great 3" travel race 29er, but very seldom have anyone interested, whereas 4" travel 26" bikes are still quite common.
That was a pretty disjointed post, I guess, but it's Sunday and it's 65 degrees out, so I'm going for a (wet, road) ride and I don't have the time or inclination to re-edit it. So that's what you get.