It's not necassarily smarter, but they did remove a lot of the "stupid." I've noticed the Grunts will notice you where previously they wouldn't. (LIke in the vents in "We've got Hostiles," where they typically will notice you before they finish the "I killed 12 dumbass scientists" line)
Also, they are no longer restricted to certain areas. (Like that one area in We've got Hostiles with the conveyer belt; I usually chuck a grenade over into the area above me to take out a few grunts before I get to that area; Apparently I missed this time, and instead of stewing in that area, they actually crossed over to the other side, so when I went to that area, I was pleasently shocked to find 2x as many grunts waiting to kick my ass )
Yeah, and also, the marines will now shoot at the Barnacles instead of just walking right into their tongues
And on hard... well..... it's.. ****ing hard now