USA Today, the Fast Food Of Dailies, Is Expanding Menu
USA Today, famous for boiling even important news down to quick bites, is changing its approach in effort to win day-to-day loyalty of regular readers; millions read it each week, but they are not the same millions from week to week, and advertisers cannot be sure who is seeing their messages at any given time, or where the readers live; to change this, USA Today has published string of long investigative articles that gained national attention; they also caused confusion at the hundreds of newspapers around country that imitated USA Today's former style of souped up color graphics, huge weather maps and near synopses of news stories; photo (M)