Training. Almost all the great downfalls and rises in programs can be attributed to a shift in the way the kids are trained. I would not attribute it to a shitty coach's personality or a lack of recruits, as William and Mary has continued to get strong talent, and many of the kids never improve past freshmen year.
Gibby, under whom the WMXC team produced numerous all americans and a top 5 team score at NCAA's XC, believed in high mileage and aerobic conditioning. For every runner, there was a strict training regimen centered around spending time on your feet. Gibby produced great distance runners, especially 5k and 10k runners, but was notorious for his inability to coach true middle distance(800m) kids. He is currently turning UNC Charlotte towards a stronger path, and definitely improving their kids, and I would consider him a very successful coach. People underestimate the importance of an aerobic base.
I would throw out that Walsh is one of those people. WM's program has declined drastically, and if you run all the numbers, it happened when he took the program over. This is what happens when you run hard everyday. Their workouts, their long runs, "tempo", and even recovery runs are fast. Training hard everyday is not the same as training smart.
I wonder why Chris Solinsky isn't making any hint of a change at WM...he arrived last year, and they got second to last at Wisconsin. This year, they got dead last, losing to the next-worst team by over a hundred points. What type of terminal sickness does WM have, and who can save it?