Mobs at your level have a 5% base chance to be missed. Hit rating scales with level, so the higher level you are, the more hit rating you need to overcome the 5% base chance to be missed.
If your hit rating reduces your chance to miss by 5%, you basically won't miss mobs of your level.
There are numbers out there on this site and others that will tell you exactly what hit rating you need at level 60, 70, and 80 to be "hit capped".
This wiki entry was made when level 70 was the cap, but the math still basically applies:
http://wow.allakhazam.com/wiki/hit_rate_(wow) If the link is not working as intended, insert the final ) (close parenthesis) that is not appearing correctly for me for some reason.
Edited, Nov 24th 2008 3:43pm by jeromesimina Edited, Nov 24th 2008 3:43pm by jeromesimina Edited, Nov 24th 2008 3:46pm by jeromesimina