Originally Posted by Scarizard
I think something that makes a champion successful in the jungle is how much that champion values - and can thrive - being in Fog of War. In Rengar's development, we knew that he didn't have a competitive clear time, but he also didn't need it - his lane-ganking (with Thrill of the Hunt) or just basic sidelane ganks utilizing his passive along with Empowered Bola Strike made him incredibly strong there.
Kha'Zix, Elise, Diana, and Vi as well all had a significant number of jungle tests - even if we didn't expect that position to be their primary/popular roles, they are all champions that have strong dueling/cc and prefer to get the jump on the opponent.
Zed is a champion where i'm probably the most happy with how his jungling turned out - CertainlyT and i got to work very closely on Zed, and at the time he was a poor jungler at best. He was functioning as a solo laner just fine, mind you, but when he was only a laner (and balanced to be one) he was incredibly frustrating...on demand slow (at the time it wasn't tied to mimicked shadow slash) and potent ranged poke + super escape made laning against a Zed so frustrating some of us didn't even want to participate in playtests. However, opening him up to play in the jungle allowed us to not only drain some of the strengths of his laning, but also increase his mobility/trickiness across the board - the Zed player was casting W more often and jumping over jungle walls, and the opponents had specific windows of time they would be expecting him within the laning phase, which ended up just being more fun across the board.
Of the champions i've mentioned, i think their jungles are quite potent - but almost all of them have to skill/build/play differently to account for their new role. Elise forgoes a sustained damage build and prioritizes raw damage and approaches with rappel to hit-confirm a cocoon into instant-kill. Vi, Zed, and Rengar will all stalk opponents from shaco-esque angles of approach.
tl;dr IMO, the more a champion can make use of burst damage/mobility from the jungle, the more potent that champion can be played there, whereas heroes like Garen/Darius who favor accruing advantage over time and attrition-based combat don't get to exercise the full extent of their strengths. I believe junglers are less 'Can this person jungle?' and more 'Why do you want to jungle them?'
I think this really hit the nail on the head.
Some champions are jungled because you want them in the game for late game team synergy but they can't lane very well. The jungle is a way for them to get levels and farm in relative safety. Some champions are jungled because they have such fast movement and clear times that they get more farm and xp in the jungle than in lane. Some champions are jungled because they are very good at preventing the enemy jungler from getting farm, even though S3 has made counterjungling much harder because the good camps - wolves and golems - are too deep in enemy territory to be stolen safely. Finally, some champions are jungled because they can gank very well and help laners win against their opponents.
Most jungle champions bring a combination of those factors to the table - fast clears, good ganks, late game control/damage, but sometimes a champion does only one of those, but does it incredibly well.
What makes a champion a jungler? The ability to benefit from the difference between jungle mechanics and lane mechanics. Some champions are horrible in lane but great in the jungle. Most champions are better in lane but can be jungled if you really want them on the team but there isn't a lane available. And some champions simply have mechanics that make clearing the jungle impossible. In S3, Leona is a good example of this. I think Alistar was supposed to be as well, but I think his jungling in S3 is actually much stronger than late S2. I look forward to the buffs to his movement speed and W range so that I can stomp with him again.