These are the ghost quests I've found so far:
1. Underwater in Azshara. Just die in the zone and swim around, you'll find it eventually. It's a dead gnome in a full underwater diving suit. Gives a quest to go see the Gnome engineer in the sunken ship in South Azshara towards Orgrimmar. Seems like the series is unfinished though, no new quest once you talk to the Gnome engineer.
2. In Western Plaguelands, in the Hearthglen keep. The ghost paths around the entire keep so just keep looking. Gives a quest to speak with Tirion Fordring, decent blue item rewards. Think the ghost was a member of the Silver order or whatever, it's been awhile.
3. In Darkwhisper Gorge, south area of Winterspring. It's a blood elf ghost, she wanders around three of the little areas/tunnels. Gives a quest to go speak with Donova Snowden, the quest text makes it seem like the starter for a key quest to get into Hyjal. Dead-ends at Snowden though, so probably not in yet.
4. In Silithus, in Hive Regal. It's a bug ghost. It's up on the wall of one of the big hive areas. Gives a quest to give some information to the Commander of Cenarion Hold. Might have been a series during the AQ gate opening events, seems to not go anywhere much now though.
5. In Stonetalon Mountains. It's a wandering treant ghost in the Charred Vale. Gives a quest for a little treant non-combat pet. Think there is a level cap on when you can get the quest though, could only get it on a level 21. Might be horde only too.