Heroes. A good hero selection is p2w because you may not have a reasonable counter to xyz enemy. They used to sell Runes too (I believe an update made this invalid). Of course, you can get all this stuff for free just by playing 100s of hours, if you have the time/will.
Champion advantages can be mitigated with playstyle and itemization. You can do well in League without having too many characters; Champs aren't hard counters to each other in the same way Dota heroes can be. Buying a certain champion is no guarantee of victory, especially if you don't play them well. Skill plays a much larger role than the champ you play.
As for runes, IIRC you couldn't buy them with the paid currency, Only with the currency earned from playing. It's a moot point since they merged runes and masteries anyways.
It's true that this disadvantage can be mitigated but we should be clear. Riot does limit your play style if you choose not to pay (or grind to get certain champions).
You don't have the same pool of characters as other players who've chosen to pay for their favorite champions and this is clearly a disadvantage.
This is different from games like, say, Overwatch, where the entire hero pool is available to everyone (different game, sure but the point stands).
