The main story line is linear! But.. I guess a main story line is supposed to be...
Leveling only one job is linear! But... I guess if you only level one job.. it kind of has to go in one direction....
Aside from that, there is no linearity. It is true there is very little in the way of customization for skills. We could have the choice for more skills/sub-jobs/etc. But after a few weeks we would have discovered the 'optimum' build... And we would all choose those same skills/sub-jobs... So Yoshi-P just saved us a bit of wait.

Now, if you really want to mix things up, try going with class and not job! More variety. Then try to take on tough monsters solo! Or see if you can make a custom setup that is unusual yet effective for a specific task!