LoL has a lot of things to work toward which actually take a LOT of time to acquire, even if you spend $$ to get some faster. Leveling 1-30 takes at least a couple weeks if not longer, depends if you use XP boosters or not (purchased with $$).
Once you are 30 you still have to acquire champions, there are like 112? Champions now, you can use in-game currency or $$ to buy champions. It takes a long time to save enough in-game currency to purchase champions, so unlocking them all becomes a long-term goal.
Runes are purchased exclusively with in-game currency which makes you decide if you rather purchase a new champ, or acquire some Runes that could make you fare better with whatever champions you have access to. Runes are also expensive and it is recommended to get 3-4 different rune sets made before spending your in-game currency on Champs. Runes will take the longest to fully unlock, if that's your goal (it will take years). You do not need to fully unlock runes, though, as there are the "must-haves" and the "probably never use unless I want to experiment" runes.
Beyond that there are skins which are only purchased with $$. I think most people prefer to collect skins as their "end game".
Oh, and as Draegan mentioned, playing Ranked is where you actually progress and "level".. or "delevel" in my case.. lol.