People please check your expectations of things that they might change for realism. These zones are not gonna change to transition better.
This is not the point of this topic, OP is not saying "I want better transitions!" LOL he is saying I want a progression of each zone to create the sense of accomplishment as you adventure along the road from 1-50. This is a major problem and something they better fix/adjust w/e. They could create all new expansions or they could just fix the mob placement. Make a big world map and organize the layout of mobs to create natural progression. If they kept the 1-30 mobs as Richard said broken into subsections within the 3 main regions so that as you adventure out of Ul dah and take a right hand turn you don't run into lvl 90 fire drakes. This is just shitty planning and design.

Solution
Central Thanalan: 1-10 [marmot,coblyn,WORM,bees]
east thanalan: 10-20 [dodo,puk,spider,ladybug]
west thanalan 20-30 [goat,vulture,beastman(ogre?),cactuar]
--------------------------------
coerthas: 30-40 [antelop,wolf,piesty,bird]
etc etc..

In order for this design strategy to work they would need to make a few changes to other systems to promote the idea of party play and encourage you to group up and adventure in this fashion rather then just leve linking from camp to camp.

Solution
-Add chain bonus for efficient killing OUTSIDE of levequests
-adjust sp caps to promote killing harder mobs (remove 10 lvl gap as max)
-add basic party search/seeking feature
-add a form of transportation outside anima (mounts system) to allow players to travel to camps and explore