>/var/adm/wtmp
Is basically saying "redirect nothing's STDOUT to the file /var/adm/wtmp", which is essentially the same as the longhand "cat /dev/null > /var/adm/wtmp" - this is a fairly common method of truncating a file down to 0 bytes.
As for the list of files.... I only administer HP-UX and Linux, so one of the AIXers here will be able to give more info, but for a start anything under /var/adm (or Linux - /var/log) is game. Also, keep your beady eye on /tmp and /var/tmp as these can quickly fill filesystems if not managed properly. Also keep an eye on wayward mail spools filling up (some /var/mail/* others /var/spool/mail/*). I should probably say that I'd rather use some form of log rotation mechanism so that logs are archived x times, rotated and removed (such as logrotate under linux), rather than blithely wiping things out.
Unfortunately the things that fill up systems the fastest are the users
Cheers
ZB