First off, the farm I worked on was fully organic, so there was a ton of work to do, and to do the work we had 6 residents, in addition to a series of day laborers to do it. I was a resident, but the latter is what I think the campaign is endorsing. We had ~12 acres of produce in fields/greenhouses and a commercial kitchen. We ran a CSA program, sold to small coops and independent grocers, and set up at farmers market - total gross was around 600,000, growing at around 50%/yr (estimated max production was about 1.2 million gross)
Not all that information was necessary, but it's kind of interesting.
The obvious thing keeping people from working on a farm is the work. There's not a lot of beating around the bush there - it's hard work, you're out in the sun, it gets up above 100 REALLY easily in the greenhouses and occasionally in the fields, there's a lot of bending over and kneeling down - which I think we are becoming worse at as a species, and it can get really dirty.
Complaining about any of those obvious things, will get you branded a whiner - and farmers HATE whiners. One whiner can destroy an entire day's labor. I'm not even close to kidding - a negative work attitude is a problem in most places, but in a situation like this, it's absolutely toxic. Whiners on our farm rarely came back after lunch either by their own volition or strong suggestion (it usually didn't take much.)
Which brings me to my next point: it's surprisingly easy, at least in my experience, to get fired from farm labor. (no, I didn't get fired) Farms need labor, but they generally run on really tight margins and can't afford to lose money on unproductive labor.
Unfortunately, another thing that consistently leads to unproductive labor is something I tend to do a lot of - talking. I arrived in rural Wisconsin eager to find out people's life stories (which I assumed were going to be more interesting than many proved to be) only to find out that, for the 10+ hours a day we were working, I was pretty much barred from talking. Due to my residency, I was allowed a little more leeway with this than the day laborers, but sometimes I would be having a great conversation with one of them without thinking about it and they would get fired at lunch. (I felt really bad about that)
But the work on a farm can be incredibly rewarding - it's work that you can look back on and think you really accomplished something. Not to mention a strong sense of connection to the food chain which I think is sorely lacking (to our serious detriment) in modern society. I would recommend it, but there's one more thing that could be an issue for the average starting in this line of work: safety.
Many of us grew up in absurdly safe environments with anything of any particular risk or danger generally abstracted out of our realm of awareness or relegated to a distant location. Mike Rowe had a fantastic bit about this in his presentation to the Future Farmers of America. His speech, on the whole, was similar to his TED talk in some ways, but he drove it home with a slightly different point. He (to uproarious applause) suggested "Maybe OSHA doesn't have it right; maybe PETA doesn't have it right..." He went on to suggest something that I kept in my mind every single time I started up a roto tiller or rode around on a fully extended forklift fastening parts of a greenhouse:
"The second you think somebody else cares more about your safety than you do... you're in trouble"
The best part is, you can substitute in other things for safety too. (success, future, children, equipment, code, etc) But this is what we have a hard time with in our culture. People don't want you to get hurt, but on a farm, there's not usually a guy whose job it is to go around making sure everyone is safe - it's everyone's own job. You may not think it, but this can be very frightening. And it extends into quality of work - if you're only doing a good job so someone will come by and say "man, you did such a great job weeding that onion patch for 9 hours" you're going to be even more disappointed than you were when you found out you were going to have to weed the onion patch for nine hours.
I guess I'll call it here, as this is getting out of hand, but don't be so quick to call the unemployed wimps - it's insanely difficult work (gets better after a week or so) and it's a very daunting lifestyle that most comfortably employed people would similarly poopoo.
Also, there's a surprising amount of paperwork involved in farming (especially organic certification) and I've been brainstorming a way to use technology to alleviate some of that burden - if you're interested, email's in my profile.
I don’t think this is true, in general. Hard manual labor, combined with poor diets and poor healthcare left our ancestors with wrecked bodies by age 45–50.
Well, you might be right that overweight people can’t bend over or kneel down as easily as fit people. But the main thing stopping fit people from kneeling down a lot is that it's unpleasant (and we have neat inventions like ubiquitous chairs), not that we're less inherently capable than in the past.