I downloaded League of Legends a couple of days ago, and from what I understand, it adopts a Free 2 Play model. But what does it mean at the end? What kind of limitations or penalties do I have if I decide to not spend money on it?
League of Legends' Free-to-Play model allows gamers, paying or not, to enjoy all of the core gaming experience.
So every element that actually makes a difference in the gameplay can be acquired for free (champions & runes). There are more than 60 champions in the game and they are locked for you when you create an account; you buy a champion for a given price with in-game money (IP - Influence Points) earned by playing the game, or you spend Riot Points. Runes give you a small boost in-game, and can only be bought with IP.
Riot Points (RP - acquired spending real money) are used for:
In short, not spending money means you will take longer to unlock all features (which are not required for having fun with the game), you will not have skins, and your Summoner Name is fixed.
In League of Legends, real money gets you two things: characters (which you can unlock by playing anyway) and skins (which are cosmetic only).
You cannot spend money to get an advantage. Everything the paying customers have, ('except, again, the alternate skins) are attainable by those playing for free. True, it may take longer, but you're getting the game for free here -- Riot's gotta make their bottom line somehow! ;)
There are no penalties for never spending a cent, nor paid-only benefits.