Okay, I'll bite. So I've only done one trail ultra (Chuckanut 50k), but it was enough to let me see how different this type of running is compared to a road marathon. It's a totally different ballgame - and while it does come down to some level of talent/aerobic fitness there are some pretty specific skills (in nutrition, not falling and breaking yourself on rough trails, staying mentally in it for hours on end, etc..) that come into play. I used to laugh at the idea that people would mention walking or "power hiking" up a hill during a race...but there I was about 21 miles into Chuckanut walking up Chinscraper hill, falling on rocks and climbing on all fours at times. On the most technical trail section, about 6-8 miles in the middle of the race, I thought I was going fast since I was breathing as hard as I would running 5:20 pace on the roads - it ends up that Max King and I averaged 8:20 pace on that section...thats how rough the trail was...not like road racing or cross country at all- and I grew up running on trails and I think Max did as well (I mean, we're from rural Oregon- there's nothing else to do out here!). Of course Max is way faster, more athletic and has actually proven his worth at real ultra distances as well as on the road and track.
Finally, the pain was pretty real and brutal at 50k - much like a marathon when you are hitting the wall. I can't imagine what kinds of bad things can happen to you when you have to go 50 or 100 miles.
This is natural running - pure and simple. I hate Vibrams, but I'll embrace running on soft natural surfaces, up and down hills, with minimal footwear...that makes sense and it's a lot more scenic and fun.
-Tier D road marathoner