It's true that Vah'jir quests are more interesting but I found myself so uncomfortable with that aquatic environment that I quickly moved to Hyjal

About phasing, it can really be annoying at times, I've even seen quests were each toon was in her/his own phase, for example killing the same elite mob at the same time. Sometimes only one of them would get the credit in the end so I had to repeat the quest again. This mostly happened in Uldum and twilight highland. What encouraged me to continue questing even after I reached max level were these appealing reputation and blue quests rewards.