And no, it's not the amount of gap closers.
When looking at the list of champions, and sorting by release date, I've noticed that the earlier champions tend to be a lot more cartoony. We have a homocidal but cute yordle assassin, a little boy riding a yeti dude, a samurai in a goofy glowing outfit with several eyes, a scarecrow, a little girl that summons a flaming teddy bear, the works. Then we get an old dude in a turtle-pace plane and a hulked-up purple doctor. Heck, even Cho'Gath, who's supposed to be a terror, looks silly. And then we have a crying mummy and ROCK SOLID, can't forget those.
Of course, there were serious champions back then, like Morgana and Jax.
However, looking at the newer champions, it seems that Riot has grown further out of their 'cartoon' style. If I start from Xin Zhao and later, which is when I joined the League of Legends, there's a distinct lack of cartoony champions. The closest to a cartoony style that I can see are Trundle, Rumble, Fizzs, Ziggs and maybe Viktor.
Is this bad? Not necessarily. But I found charm in their older champions, just because they were so damn goofy. Giant armadillo that doesn't say anything other than "ok". Big Spartan dude with his face completely obscured by shadow and helmet. Serious-mode champions like Talon and Nautilus sell like hotcakes, but what I'd like to see soon is a goofy-looking champion, like Riot used to create.
Make a goofy champion, Riot, and you will get all of my money.