I'm with Barb - I like the aft best.
We've been mid-ship, forward and aft and I definately like aft.
Pros for forward:
Easiest access to the spa, adult pool, Walt Disney Theatre, and entertainment area (Beat Street/Route 66).
Cons for forward:
farther away from resturants and other food (Topsiders/Beach Blanket)
more motion of the ship felt (but not really that much more (although when we were forward we were on a very, very smooth cruise and I couldn't feel it much).
Pros for aft:
Very easy access to the drink station on deck 9 and the rest of the food (love going up the secret stairs into Topsiders.
lEasiest access to the Mickey Pool
Close to the deck 7 secret deck
For many (but not all) the safety drill is inside at Animator's instead of out in the hot sun.
Cons for aft:
Long walk to the spa/adult pool (but good exercise)
Some vibration felt from the engines
As you can see I really focus on the pros to each because in my opinion they are both great locations (as is mid-ship).
Hope this helps!