The wide selection of hero units is a major plus, but it's variety of choices is a bit overwhelming, particularly if you're the type of player who has a really hard time finding a class and sticking with it (like me). Riot Games has implemented a number of tools to help you sift through the numerous types in order to help you find something that will fit your play style, which is a major plus. At the same time, the number of classes is so immense even with a filter pointing at a certain type; you'll want to experience everything. Luckily, the game is well designed and something you'll want to keep playing. Riot deserves a lot of credit for managing to find a way to introduce so many hero types while also keeping them competitive and fun to play.
The basic mechanic of choosing a unit also hints at some of League of Legend's hidden depth and challenge. Choosing a character introduces an interesting meta-game similar to Magic: The Gathering where you're not just looking to play as a character that suits you, but also something that could potentially offset whatever class your opponent may choose to play. I've yet to see an all-powerful class or classes crop up, but finding the right balance is sometimes more interesting than the core game (at least, it was for me... but I'm a Magic nerd who loves that sort of thing).
Defeating enemies earns money and experience you can then use to upgrade your Champion. One ability point is earned for each level, which can then increase your Champion's abilities up to five times. Money is used to purchase new weapons and items for your Champion.
The core game is available as a free download, which will let you jump in and start playing immediately. A $30 Collector's Pack is also available, which contains the core game as well as access to special Champion skins, Rune upgrades and $10 gift card redeemable in the in-game store. League of Legends may not be a must-play game for everyone, but when you consider the no-risk price, it is definitely a must-try game.