Riot couldn't decide if they wanted to make a bruiser, assassin, or melee carry.
His damage output is carry output. He's got steroids out the ass, which is cool.
His ult is an assassin ult. It gets him in and helps isolate his target. His base stats are pretty much the same as talon, and in the general ballpark of assassins. This is cool.
This however, has a problem. Assassins aren't sustained damage dealers. Noc is ultimately dependent on dealing sustain damage, and of melee carries, say, tryn, yi, and yasuo, noc has the worst combat mobility, the worst survivability, and the worst damage. Yes, his damage output is high, but compared to these other champs, it still sucks. Noc consistently dies to other melee damage sustain damage champs.
He has lower burst than pretty much every assassin out there, as well. He also lacks anything resembling an escape or combat mobility. About all he has is sticking power for things that don't have external peeling
Noc's teamfight is shit. I'm not even going to try to say it can be good. It can't. It's terrible. You have to build tank on a champ that begs to build damage through mid game if you want to survive. Most games where I get fed on nocturne, as soon as teamfights hit, we consistently lack initiation, since a squishy noc sure as hell can't. In fact, the only time noc is usable as an initiator is if there's an orianna ball sitting on him. Tank noc can initiate, but that's like mundo running in and his team following. That's really not that good. He can't peel. He can't stick in teamfights. He basically sits there getting kited, and if he built tanky enough to survive, his damage isn't threatening enough for it to matter unless he's ridiculously fed.
His cc IS FUCKING AWFUL. Really, buffing its damage? Why. That makes no sense. That's not what it's used for. Ever. Riot goes on and on about how the like keeping champ strengths and removing power that doesn't add anything meaningful is a cool way to nerf champs, but then when they buff noc, they add this shit instead of actually enabling him to use his strengths. How about increasing the leash range with level? How about decreasing the timer with level? I'm sick of every champ with a built in fucking dash just getting away for free. Noc can't touch champs with dashes. Seriously, his E may as well be a dead spell.
His jungle sustain is shit. I don't know why his passive is so gimped. It's honestly really lackluster.
The only thing I'm honestly sure about Riot's stance on nocturne is that they want him to be consistently overshadowed by everyone who does more than he does by also letting them do those things better. Look at pantheon. Why doesn't he do better than noc? Long range engage? It's AoE, ccs, and is untargeted. CC? it's a gap closer and it's inescapable. Burst? Panth actually has that. Panth even gets 2 blocks instead of 1. Mind you, auto attacks aren't as impactful as a stun, but at least panth can build 
and not have it get used if noc blocks with his shield too.
The only thing I can think of that riot likes about noc is that he destroys isolated targets and has godly sticking if he isn't cc'd. That said, only about half of noc's kit actually helps him with that identity. His passive is half assed sustain that only exists to help his jungling. His E is, between its leash time and range, just bad. His W is good, but it really needs to stop playing Three Stooges with . His Q is good. It really fits what the champ seems to want to be. Perhaps a rework based more around his Q would be interesting. His ult could be good, but on his current kit, it doesn't do enough.
I don't really know what to do with him. I want him to be a worth while pick. I like the his theme, and I think that his theme has a lot of potential. A lot more potential than is currently utilized. I think everything from his kit to his stats to his play style are scattered about, with some aspects saying assassin, some saying melee carry, and some saying bruiser. I think he needs at least one of these removed from his kit. I think the most interesting thing that Riot could explore is to make him a mobile melee carry that can turn teamfights into skirmishes, allowing Nocturne to better create an environment where he thrives.
, which are both squishy base stats, except unlike Yi he doesn't have untargetablilty, damage reduction, or slow immune, which is why he is usually built like an off-tank instead.
