Recoil mostly. They stated that a .40 is "snappier" even than a .45, which makes some sense to me being that it is a higher pressure round. But like i said i have slim to none experience shooting a .40. This info was coming from sources that shoot several thousand rounds a year and MANY people on this site would trust very well.
I can attest to this personally, on a limited scale. I've shot a G30, G36, G23, G24C, XD40 Standard, and a XD45 Tactical. In my hands, it's readily apparent that amongst these semi's, the .40's recoil is sharper and of shorter duration (i.e., "snappy") than that of the .45 cartridge. I prefer the .45 handguns partly for this reason, though I'll be the first to say the G36 is quite the handful