The acheivement is triggered from reavealing all the parts of the map - however there are usually serveral mini-zones that can reaveal a specific part of the map, hence its possible to still get discovery exp even after the whole map has been revealed.

The exp rewards from exploring are ok, but don't expect to be gaining multiple levels with it at level 77...depending on how much you've explored, you'll probably get about 1/3 to 1/2 a level. Just watch out for phased areas, as they will break follow on your alts.

Mapster is a good addon for this as you can toggle between complete and incomplete maps to see where you have to go.

Cheers,
S.