For me it's been a two-parter: what made League significantly less fun for me was Preseason 6, and its subsequent change in the pacing of matches. It felt like a transition from Team Fortress 2 to Call of Duty, where fights that once gave me enough time to devise cool tactics started ending far too abruptly, mainly because everyone started dying too quickly. Comebacks became less likely, and generally it felt like my matches went from this interesting back-and forth to a coinflip, where whoever fell behind effectively turned into a helpless bystander as their team got crushed. After that, I started playing League significantly less.
More recently, what's been turning me off is what I perceive as a stagnancy in League's design, and Riot repeating a lot of the mistakes they've committed previously. League has a lot of systemic issues that have required urgent attention since its earliest seasons, but these problems have remained unaddressed still. When Riot does announce that they're tackling a problem, their response has tended to miss the mark, often for reasons they should have been able to predict. Runes Reforged, for example, advertized itself as a rework to the rune system, but really was yet another seasonal mastery update with a fresh coat of paint, and as such it has ended up with the exact same core problems as its predecessors. When the Juggernaut Update launched, I loved the intention and freshness of the idea, even if the execution was largely unsuccessful, but by the time the Tank Update came around, I felt like Riot should have seen it coming when their reworks fell flat, as no previous roster update had truly worked. Additionally, several newer champion releases and reworks have felt pretty samey, even when they turn out utterly catastrophic: Zoe in particular quickly became hated among players, and so for reasons that had been pointed out in other parts of the game, such as Nidalee's nuclear spears (i.e. her one-shot Q, arguably an even worse version of the effect), the irritating stun + nuke combo on any burst mage, and any kind of RNG. Her failure was entirely avoidable, yet somehow we still have that champion in the game. It's this refusal to learn from their past mistakes, along with a design philosophy that has become increasingly inconsistent and more focused on generating PR at a particular moment, that has put a damper on my hopes for League's future. I still do believe that League is an awesome game with practically limitless potential, and that there are fantastic people at Riot who are working hard to improve the game, but I'm not entirely sure if those people are being empowered to do the right thing, or what the direction even is for League at this point.