The general consensus is that season 2 and season 3 were the best seasons, snowballing is too heavy, and that solo carrying is impossible.
The general consensus is also that tanks are invincible (a symptom of season 2), Fizz, Zed, and Yasuo are cancer (2 champions w/ Yasuo in that same archetype that essentially ran the game in S3 by getting one kill and then snowballing the game to a solo carry), and that botlane solo carries the game, which is ironic because solo carrying is supposedly impossible.
I understand that the site isn't a hive mind and that the boards has many different opinions, but if you take even the briefest look these will be the most upvoted posts consistently.
I think the issue is two-fold. One part is an issue that I had when I grew out of RuneScape. You look back at 2005 when everyone was playing and the game seemed really magical. A bunch of strange people in a strange world where you could do everything you wanted. Anyone who played the game will tell you it was fucking great at the time. But once the realization of the game's limits hit you, especially how much you had to grind, it just turned people away. They learned the game and didn't like having to spend hours and hours to get a 99 and/or pay to get memberships and so they got frustrated with the game. This is compounded by the fact that not knowing the nuances of League is a lot more punishing than RuneScape. If you don't know you're being inefficient in that game, you might never find out, and if you do you might not even care. In League, you can get 2 kills on a guy and still be 1 level down on him because you don't know how to manage waves effectively, which gets frustrating.
The second part is the meta-shift. Once core parts of the game that support your playstyle get changed, you feel alienated. That, combined with frustration, are probably the #1 cause of players quitting the game. You see it often in card games because people don't want to keep paying to stay competitive, but if we go back to the RuneScape example this is analogous to EoC, Squeal of Fortune, and Effigies. This is particularly easy to do in a game like League where there are so many nuances if one or multiple become common knowledge people who don't watch streams or browse Reddit end up at a severe deficit in terms of game knowledge.
This is why you often see lower level/ranked players view previous seasons through rose-tinted glasses but high elos will talk about how the game was just as flawed back then too, because they know the various ways the game was abused by the people who knew the game well. I'll even share some of my worst experiences as an example.
Season 3 - Fizz and Zed were everywhere. Both had even more damage than they have now. They get one kill and snowball the lane into solo carry victories. Supports literally bought a pair of boots and then stacks of wards. They were often the target of said midlaners who would get one kill and snowball lanes.
Season 4 - in preseason, bot immediately had to play much more passively because toplaners took TP. Said toplaners were invincible because of defensive masteries. Also, if the enemy team got Annie support you weren't allowed to do anything because she'd instantly blow you up. For the first 3/4ths of the season games stalled out to 45-60 minutes in solo queue and it was impossible to end the game unless Ziggs/Ori was dead. In LCS, 60-80 minute games were common. Riot eventually gutted Athene's and Ziggs and the game immediately shifted to Zed, Yasuo, and Jayce everywhere. I know how much Boards loves their AD mids. /s
Season 5 - juggernaut rework. I feel like enough people here remember GP and Fiora running over games until Garen outplayed everyone by letting them kill him and then his villain mechanic would proc on them. Also 60% winrate Mordekaiser.
Season 6 - League of Dynamic Queue
And despite that, I still have lots of fun because I understand the game and enjoy the game for the game. I can see the flaws and play around them if I don't like them and I have to accept that there are people that will abuse them because everyone wants to have that high elo icon. It's been this way since the dawn of the game. Hell, as far back as I can remember I still know a guy like that who hit Challenger playing AP Soraka mid. I think that this mindset has simply almost entirely diffused to the lowest elos in ranked play.
tl;dr Boards users are not adapting well to newer metas, so they blame many factors other than themselves when they're the ones who aren't becoming as efficient as their peers. Boards is right, the game is a lot more enjoyable when nobody knows the right meta, but you can't expect people to unlearn the game so that you can have fun again. You're not gonna get that feeling again, and boards' obsession on past seasons is flawed in that it focuses on the numerical value of the season rather than the freshness of the game and the players' naivety.