LoL is like DotA2 (while Heroes of Newereth is like Dota+, basically better graphics, more stablle, but largely the same heroes... it's also a one time purchase instead of LoL's microtransactions). While LoL is not "free" in the sense that to unlock content you either need to shell out or grind, you do get 5 starting heroes and if you have a bunch of friends grinding isn't hard. It has better graphics, more maps, heroes (all original) have more skills and it's less swingy than DotA. You also have "summoner skills", and a skilltree for your account. Your account levels up and you get new skills that you can use for your heroes. For example, there's a blink like summoner skill, a heal, a WW, etc... There are also "runes" which let you add +stats to your hero, letting you customize them to the extreme.
While that all sounds very nice, in practice it means you basically don't unlock the "full game" until you hit lvl30. Many heroe builds/strategies are simply not available until you hit a certain level (you can't jungle from lvl1 effectively for example, some heroes are a LOT better with a full dodge rune set, etc....). Mismatches of lvl30 vs 1 don't happen often because of the matchmaking though, so it's bearable (again, as long as you have same level friends... having a level1 in a pre-made of 30s is begging to get ownd.)