There isnt one magical level when the game changes, because the changes people are thinking of arent always the same. As a thaumaturge you started out in Ul Dah. Some people think the game gets better when the story sends you to the other 2 starting cities and you suddenly have a lot more to explore. Some people are thinking of the first set of dungeons as a turning point(or they might have one dungeon in particular that they like and point to). For some people its the first trial. For some its a story related beat(there are several great ones). For others its based on when they start getting more moves and combat becomes more complex. There are some who are thinking of the expansions. Some people are thinking of a side piece of content they love(Hildi! Gold Saucer! Glam and Gpose!). The game is deep enough to support many answers to this question.