Jungle issues seem to me to be split between unresolved issues from last year's preseason and then new issues from this years preseason.
Jungle is hard as hell on initial clear. This has been an issue since last preseason. It was fine-tuned a little bit (smite given second charge, values of monsters changed), which made it possible for at least a 3 camp clear with a leash from bot in low elos. High Elos saw the difficulty of jungle entirely camouflaged by the "Double Jungle" start, where Jungler had a tank for a camp and a half. With an earlier start time and knocks to resources generated by killing monsters and a nerf to teleport, the double jungle doesn't seem to be highly valued. Earlier start time also means 2nd smite charge isn't available as early, which creates an issue with having breathing room on first clears.
Healthiest/Safest clear I've found is to start red side, get leash on Krugs (smite) -> Raptors/Chickens -> Red Buff (Smite for HP). Some champs will barely survive, others (Bruisers with Sustain) will be healthy enough to gank or run to blue. The issue with this clear is it's health efficient over time efficient. Running from Krugs -> Chickens -> Red is slower than Krugs -> Red -> Chickens. This puts you behind in XP/s, so if a solo laner dies during this time, you'll soon find yourself in that dreaded situation where enemy solo is L6, your Solo is L5 and you are L4, meaning enemy Solo is probably going to have a window to free farm or to take another shot at your laner without you being able to do much. Proposal: Make Smite generation charge start earlier, your Red can be smote as second camp for HP return. You might even get 3 smites on first clear. JNG can now clear a few more camps safely or look to gank.
Harder initial clear is coupled with lack of money. Red/Blue/Green Knife got more expensive and lost gold on kill. This equals less money and a slower game for the jungler. Items seem to have gotten more expensive across the board, but I'm not even getting those, I'm struggling to build the item that allows me to efficiently clear jungle. Increased cost of potions is something I wince at on every back, as I can't get as many as I'd like. Proposal: Gold per kill needs to be added back on knife. Seriously, nobody has even let me have scuttlecrab since knife $ change.
Although items from a lack of money perspective was addressed above, some of the new item's themselves are more of a hindrance than an aid. Not being able to upgrade my jungle item on a bad first clear is simply demoralizing. Not grabbing a potion or just 1 potting early is frustrating. I hate grabbing the second Tier 1 Jungle Item instead of a full-on upgrade, it's always like I limped home to grab the inferior of the two items, as I have already purchased the best Jungle item for my champion on game load. Proposal: Roll the two Tier One Jungle items into 1 item.
Finally in the early item category, Refillable Potion is terrible to start with and Hunter's Potion is a lackluster option and a slow play. 3 Health Pots is 250 HP more that a Refillable Pot on first clear. I need that HP. As a side note: It seems Riot wants me to start jungle item+refillable, backing for hunters pot, theoretically giving me more sustain. Hunter's pot is laughable, and wastes my time in getting to a jungle item that can keep me safe or scrap with enemies. Proposal: I got nothing. I hate the new Hunter's Pot, I don't even feel it should be in game. Ganking Junglers should be able to get items to gank, most farming junglers have a kit that allows them to farm pretty well already - or they take the knife designed for that. Hunter's Pot isn't something I see as needed. It's going to have to be incredibly strong to be relevant. Wouldn't mind seeing charge system added to Green Knife. Like, killing 3 large monsters gave you a ward back. That'd be sweet.
Finally, the new masteries have hurt jungling. Under the old masteries, almost every jungler got: (Defensive Tree) Tough Skin: -2 DMG from Monsters, VetScars: +36 HP, Juggernaut: +3% HP, (Offensive Tree): Fury: 5% Attack Speed, Martial Mastery: +5 AD, and usually a point in Sorcery (CDR) or Butcher (+2 dmg). These easy to reach points gave you a little bit of sustainability and a little bit of auto attack clear/dmg spam clear.
The new mastery system makes you take choices that see no real benefit, or you choose between stuff that can make the game more fun PvP wise or make the game easier PvE wise. For instance, Resolve Tier 2 has Explorer and Toughened Skin. One is extremely helpful for giving you speed, but the other helps you live. If you decide on the mobility skill, you'll be able to make the most of more opportunities. But the other might help your clear. When I'm playing a laner, I always feel fine with the skill I've selected. When I'm playing jungler I always feel like crap cause having both would be beyond helpful. Then there are situations like Tier 3 Ferocity. Vampirism or Natural Talent. Natural Talent's stats are nonexistent early and weak late. Vampirism scales better with higher damage. As a Jungler I need stats early cause I start fighting as soon as monsters spawn. Vampirism is what I typically take, giving me 2% Life Steal on like 65 AD, or about 1 HP of Life Steal. This is basically nothing. The Mastery System is riddled with choices that are meaningless or meaningful in such a fretful and infuriating way - like Sophie's Choice, League Jungler Edition.
Also, I miss Sorcery in ganks. Starting 5% CDR from Sorc and 5% from blues gave me another cast combo during a gank. It was easy to get and let me make the most out of my limited interactions with the laners. Keystones outside of Resolves' Strength of Ages and Bond of Stone don't help in Jungle AT ALL.
Proposal: Masteries need to feel better for Junglers and more options need to be added, particularly in areas where you have 2 scaling masteries to choose from (like an early-ish game mastery needs to be added. Perhaps 5% Cooldown to T3 Ferocity). I don't have all the answers to the new mastery tree, just a strong dislike for it. Please reply or post Mastery Tier Issues and Proposed Solutions. To be honest, I feel overall this Mastery system, with masteries that are unique with active triggered effects and very powerful opposed to minor, accumulative and subtle, is going to be hell to balance. I guess someone at Riot likes a challenge.
So, TL:DR: Earlier 2nd Smite Charge, $ On Kill From Knife, Remove Hunter's Pot as line is Trap (Lets you buff Refillable for laners potentially), Take another look at Masteries with an emphasis on creating early game talents and reducing frustration for Jungler's speccing out their champ, seriously read the above 3 paragraphs.