Sitting just hurts.
I'm young and in good health, so I haven't given much thought to the long term health implications of too much sitting. What I can anecdotally report is that the more time I spend in a chair, the worse I feel. The relationship is almost linear. Not coincidentally, the realization didn't dawn on me until after I graduated from college and began working full time. Before graduation, I spent a lot of time on my feet walking to and from class and to work, since I had the good fortune to have an internship within walking distance of my college campus. This was only eight months ago, but the change in my energy level is very discernible. I can only imagine how pronounced the effect would be when magnified over the course of many years.
But that's just the job. I imagine that for most of the Slashdot audience, sitting at work is often unavoidable, unless you can afford a nice walkstation setup. What about when you're not actually at your desk, though? Usually, you're still sitting, even if you're going somewhere.
The biggest sitting problem (for Americans, at least) outside of work is that our cities, our jobs, and even our recreation is not really intended for pedestrians. I love to walk, but many places and jobs are not pedestrian-friendly. I have so grown to loathe driving that my long term plans include moving to a city where it's easier to get places by walking or riding a bike, possibly for this reason alone. Currently I live in the Dallas/Fort Worth metroplex, which I feel is one of the least walkable areas on the planet (though there are notable neighborhoods that provide exceptions). The metropolitan area has suffered badly from the urban sprawl blight, so if you're lucky enough to live with your romantic partner, chances are, at least one of you is going to have to deal with a commute. The course of my day starts off with a thirty minute drive, followed by sitting on my tush for the subsequent eight hours. Then I drive another forty-five minutes home. Unless I then drive to the nearest gym, which, due to the sprawl here, is likely to be more than six miles away, I'm pretty much stuck indoors again. Side note: a 117 degree heat index does not a happy human make. I have seen Texans drive to their mail box.
The economic forces that drive sprawl really kill the cores of cities and make life miserable for commuters. My partner is burgeoning traffic engineer, and he's taught me some of the things that walkability projects can do to improve life for both residents and businesses in a city. Suburban Nation is an excellent read on the subject. You can also check out Walkscore to see how your neighborhood ranks. It's pretty neat stuff, and I wish more people would care about this issue.