as a long time player who has taken alot of breaks i can tell you mists of pandaria wasnt bad. in fact i enjoyed it, especially the story. much better than cataclysm. on par with WOTLK if you ask me.
the boost one player to 90 is huge and i dont think its because pandaria is bad. i think its an effort to bring back those that left in previous expansions. as was said at blizzcon, players might really want to play WoD but to buy the expansion but only have a level 70 character would require you to play through 3 whole other games before you ever saw the new content. not very enticing.
the time travel aspect is going to be a little wonky. its not real time travel, its more of forming an alternate reality and using time travel to create it. the portal will change once you complete BC and turn red for WoD.
i honestly havent played more than a few free hours in the last calendar year, but ill pick this up as a time sync and to see the new content when its released. i might even get it for my fiance and use the direct to 90 on her characters that never got past 30.