Simply put, this game, while part of the Final Fantasy series, is not the same type of game as the single-player jrpg's that make up the bulk of the series.

This in an MMO and as such plays very differently. There is a main storyline to follow through to level 50, but that is only a small fraction of the things to do in this game so you can't expect to only do that as you will most definitely run into points where you will have to go do other quests, run dungeons, participate in fates, etc. to reach the required level to then be able to complete the next story quest.

As others have pointed out, MMO's are much slower and repetitious as compared to a single-player game so this might not be the type of game for you, just because you have enjoyed the other Final Fantasy games that doesn't mean that you will like this one.

Also, MMO's tend to start very simple and very slow so that players that are not MMO veterans are not overwhelmed and can learn the ropes and then it grows in complexity as far as combat, boss encounters, etc. the farther you get in it. Look up some YouTube walkthroughs of the Primal EX fights and you will see the difference in content at the beginning of the game and stuff that will come later.

However, before throwing in the towel i would suggest that you check out the other things the game has to offer. Bored with GLD, try out a different class/job. Only been doing story missions? Try out some of the other quests available, run some dungeons, etc.

At lvl 25 you have seen very little of what there is in the game.