So I did a Spartan Race recently to see what all the hype was about. A lot of people on here have wondered why more runners don't hop in and blow the 180-pound elite racers away on these courses. The answer, at least from what I gathered, is that obstacle racing (or at least Spartan Racing) isn't a running race, or anything even remotely close to it. If I had to describe it as anything, I'd say it's a speed-hiking race with lots of strength-based obstacles cleverly placed throughout the course.
Elite runners are obviously lean from head to toe, and these types of races tend to favor guys with thick calves and powerful quads, especially through ankle-high rocks and weeds on a 27% grade up an off-trail hill. A good obstacle racer's body is almost opposite of what an elite runner's body should look like, especially in the calves, shoulders, and forearms.
The obstacles are heavy enough that unless you have significantly above average strength for someone between 120-150 pounds, you won't be able to do at least a few of them. And regardless of your fitness level, 30 burpees (penalty for missing an obstacle) will drain a bit of your energy, not to mention add time to your race.
What surprised me about the race I did was that there was very little trail running involved. I'd say out of 14 miles, only 1.5 wasn't through weeds/brush, water, and mud, and up and down very steep hills. On the few stretches of open running I would zip by people, but when I'd fail an obstacle, they'd more than make up for the time I gained.
If a 15:00 5k runner could complete all the obstacles, I think he could still do some damage (top 10-15ish) at one of these races. But I think obstacle racing has enough top athletes who focus specifically on these types of races now that it's very, very unlikely someone could be both an elite runner and obstacle racer.
Obstacle racing has more similarities with cross-country skiing than running.