I have been playing the game for about 10 years now, the majority of which has been spent playing casual, non-ranked games as I want to not take the game too seriously(it's a game not a job...for me) and just have fun playing whoever and building w/e items/runes.
What makes this game fun for me is the high amount of champions, items, and runes you can choose from. You can also acquire skins with in-game currency as well as real money, with buying them being the easier option. Riot places sales on skins and changes them from time to time as well.
What makes the game not fun for me is that unfortunately Riot has decided trashtalking is not a thing anymore. You can get auto-chat banned by a bot for anything anyone finds to be "toxic" or negative/offensive.
TLDR:
Upside: Playing different champions, skins, items and runes is very fun.
Downside: MLP online allows more trash talking than this game.