1. I personally burn thru all the side quests to get them out of the way. But the choice is yours really.
2. People will expect you to have your job class and skills by at least level 34. You might be gimping your job by waiting until lvl 50 to level the sub class.
3. Level 50 crafter's are needed for melding weapons for relic quest and just plain melding anything of importance. A lot of times the weapon you need to turn in to the relic blacksmith needs to be crafted as well. You'll make some serious cash on the market with those.
4. As of yet the only other hobby on game is leveling your fighting campanion chocobo. There's no requirement for it yet.