He's talking about recoil. The 300WM has a LOT more recoil than the 308Win. Granted, I haven't fired either, but that's what I've run across the most often in my research.
For the record, I will be buying a Remington 700 5R 24" 308Win within the next month. Was originally going to get an SPS Varmint, but I was swayed by some people here to start out with a good base. And FYI, the 5R comes in a 20" with threaded barrel. It also comes in 300WM.