One problem with having a fantastic ignore system is that it can eliminate valuable feedback about a community's sensitivity to individuals and their content which would otherwise happen through reported posts or alternative avenues of complaints. This feature would clue in admins/moderators on which users the general community would like to see gone, restoring that feedback without any effort on the part of the user and thus eliminating the problem quite conveniently.
An advanced option could be to show the names of who all was ignoring that user.
Another advanced option could be some sort of automatic action against the user once they reach a selected number of ignores against them.
If the ignore system was expanded to give the option of ignoring threads, as suggested here, then that's another area where this feature would come in handy. I'm thinking that it could be added just below "Replies: [#]" and "Views: [#]", to simply have "Ignores: [#]". Seems it would take up a bit too much space to have it stated on every one, so it'd be nice if it would only show at all if there were one or more ignores on a thread.
These tools would assist in understanding at a glance the needs and preferences of a community which might otherwise go unnoticed.