So, recently (level 20 to now- 30), I've been really only playing jungle, as I love the idea of chilling in the jungle and being a fundamental part of the succes of all lanes. With that being said, I am no longer mediocre, generally not failing my ganks, counter junking well, knowing timer on camps and being there. So what should I try to work on next; what separates the good junglers from the average ones? What do the good junglers do/have knowledge of to that average ones don't?

The location of the other jungler is key, if you see them top, take dragon they wont contest it. Also try know the clear times of other junglers, if you know that they wont get to red til 3:20, consider waiting there for them, or taking it

There are a few things all good junlgers do:
1. control dragon -- like shau said, owning the dragon takes a large amount of map awareness. knowing when to initiate this is pivotal.
2. working around wards - learn how to gank by taking all different paths to lane (river, enemy jungle, through your team's lane, etc). Sometimes the long route may be the most efficient route for a gank.
3. warding - throwing a ward down at your enemy jungler's red/blue buffs allows the entire team to counter gank. furthermore, putting wards at entry points of the jungle allows you to easily predict when mid is getting wraiths and top is going for golems.
4. map/game awareness - know what lanes need babysitting and what lanes do not. if your top is getting pummeled, don't only gank once, repeatedly keep ganking till you turn the tides of that lane. with that being said, never neglect a lane even if you're babysitting. for example: if you're camping top, check their jungle for gank opportunities or walk to mid to help your mid out.
5. adapt to early, mid, late game roles - start realizing what your champ excels at through the different phases of the game. I main shaco in the jungle. with him, early game counter jungling and ganking is super strong, mid game I focus on lane ganks and dragon control, late game I split push, own the buffs and try and create mismatches in the favor of my team.

Hope that helps. Those are by no means a finite list of what makes a good jungler, but if you can do those things well you will most likely do better more often than not.