For home users it has to update the virus definitions automatically. Most endusers can't be trusted to click the update button.
Let me take that a step farther. Not only does it have to update automaticaly but but the time in between updates is an important factor. I know when I was looking around for a good AV to use that AVG updated exponentialy more often than McAffee or Norton. That may or may not ne the case today as I have not compared them in some time.

Also automated scans and what it does with infections is also a major factor. One problem I have found with AVG is you have to actualy check the logs to find infection if it is found during an automated scan. For example I picked up a virus not to long ago and my system was scanned at 3:00am. I did not know about it untill I checked the logs at 11:00am. So AVG kinda droped the ball on that one. When I turned my screen on there should have been a window up saying that there was an infection found. Hell I should have found out while i was downloading the file. Or very shortly there after. And what average ordinary every day (l)user checks their AV logs on a regular basis?

Now all this is coming froma home user that doesnt have much experience (read: any) with corporate level software.