I've killed my share of deer with 80gr. bullets in a 243 and it isn't that much different from a 223 with heavy bullets....Of course the key to clean kills with these light rounds is the same as with heavy rounds, shot placement, shot placement, but it's even more critical with the light stuff....
I'll bet there are two kinds of guys that use the 223 on deer...Guys that are very good and go the extra mile to put the bullet where it needs to go and the totally clueless that have no idea that the 223 is on the light side without solid placement....The guys here that have shared comments and pics seem to fall under the very good description...LOL
Personally I wouldn't hesitate to pop a buck with a 223, but I'd rather use a 243.....When I was using a 243 I decided I'd rather go with something with more power and purchased the 7mm mag that I shoot today....
From my experience the only time the big 7 gives me an advantage is when I'm hunting on the ground, spook a buck and have to take a long going away or quartering away shot....(I live in northern California and hunt on public land where you might only get one opportunity to fill your tag). The results of shots like that are seldom pretty, but the 7mm gives me the horsepower to drive a bullet into the vitals. I wouldn't try this sort of shot with a 223 or 243.....
Some guys might slam me for taking the going away shot, but to me it's part of hunting...On TV shows they seem to always have the standing broadside shot, but in the real world absolutely perfect shot opportunities are few and far between.....At this point I've killed around 30 bucks (that's pretty darn good in California) and I've yet to lose a wounded deer. I've had to track a couple, but things have always come out well in the end....