One thing I've noticed about a few places in regards to exploration.
Some zones you can walk one step in any direction and discover 1/4 of the map.
Other zones, you have to find that one exact spot to stand in or you won't discover the last smudge on the map that you need to get the exploration achievement.
There's something else too... most of the time, even though it's part of an already filled in map, there are still places to discover on the map!
And if you're trying to ding 78 with 1/4 of a bubble (which is a huge ass chunk of xp still) left to go, you want to hunt out every one of those nooks and crannies.
I was half-way through Icecrown, with only three spots in Sholazar and Borean Tundra to explore, when I dinged 78.
And dear lord... level 78 was EXPENSIVE for a warlock. I think I easily spent 200 gold just getting new skills.
Maybe I hadn't leveled when I hit 76, who knows.