Topic Is it worth coming back?
Yeah, quite a bit of gameplay has changed, and of course the new content has been added, mainly for 85+. You could do the trial or post in the SoR request sticky and spend a bit of free time in the game to see what you think.
The old world hasn't changed since the Cata revisions and you still have to play through that if you start a panda.
The new content is well done graphically, the panda starting quests up till you choose whether to go Horde or Alliance are interesting and flow pretty well. If you then choose Horde, Gorrosh out does himself being obnoxious (imo), but the Alliance leader doesn't do much better (at least with the Alliance toon I played in Beta). If you last played 16 months ago, you experienced Cata and the changes introduced then to the world. Mists quests involve phasing just as in Cata, but from what I have seen so far, when items are offered as quest rewards, they are usable by your class/spec (no more caster stats for melee characters).
The entire talent system had a major change and bears little relationship to the previous systems. Now each class gets the same three talent choices at each talent level (every 15 levels) regardless of spec. This means there are no longer cookie cutter talent paths each spec needs to follow and you can choose which ones are most suited to the way you play.
So, is it worth coming back? Only you can decide, whether or not the expansion is fun and worth playing is an individual choice. You use the trial to at least see the new starting areas and perhaps that will give you a clue if you make the plunge or not.