The ranking system and leveling system don't play nice with eachother. In fact, these systems make a lot of people give up on league of legends pre 30, because if you win a few games, you'll find yourself paired with players that have more summoner spells, more mastery points to spend, and some that stupidly buy runes before they can get tier 3 runes. The game already gives you zero player feedback as to what you're doing right or wrong in a game, so this on top of it makes a lot of new players want to not play the game because they think they're bad, and may not necessarily know that they're being paired with players that inherently have more power from the leveling system than they do. And that's not to mention the smurf accounts of bronzies that can't get out of their division all the way of to people in diamond that want to inform new players of their place in the universe: beneath their boot.
Overall, there are a LOT of things Riot needs to rework outside of champion mechanic interactions, or even Summoner's Rift itself. A lot of these mechanics used to work when the game was in its infancy, but now it's time this game grew up with its player base. But, no, Riot is more concerned with making new and interesting champion concepts, instead of fixing things like the ranking system, leveling system, rune system, the store half the time doesn't even work, the death recap system usually has some pretty inaccurate and ultimately useless information, and there is no good tutorial on how to play league of legends, so you get players that are in bronze and lose games because they just weren't taught how important cs, objectives, and warding are before they go into ranked. Yeah, some people learn it themselves, but others don't learn mechanics and strategies in the same way. The tutorial says, "you get gold for doing these things, you get attacked by these things, you move and attack by clicking, your abilities are mapped to q, w, e, and r, now you know how to play Ashe, you're on your way to diamond."
Yeah, this game is good in some ways; they usually keep some relatively stable balance between champions at all times that aren't big launches, and can be fun to play. However, there are some big problems with it that Riot needs to answer for.