Masteries
What are they?
Luckily they are balanced enough that even that is not an issue. Your choice of Masteries is usually a reflection on your personal preference of playstyle. Some champions are a bit limited in their selection: an AD carry for example will run 21/9/0 with little variation. Others however get a bit more choice in the matter. As Warwick, I might run down the Defense tree if I plan to try to outsustain the other guy and transition into a tanky initiator, or I could run a more Offensive build if I plan to gank and go all in for the kill.
League of Legends players can edit their Runes and Masteries and Runes to customize any champion as they see fit. They provide maybe a ~5% boost to a champion's early game power, waning to somewhere around 2% later on. Since everyone at level 30 has equal access to masteries (and to a lesser extent the runes) there's not really a problem with balance. If the Offensive page is OP then everybody uses the Offense page and nobody gets a real advantage.
Luckily they are balanced enough that even that is not an issue. Your choice of Masteries is usually a reflection on your personal preference of playstyle. Some champions are a bit limited in their selection: an AD carry for example will run 21/9/0 with little variation. Others however get a bit more choice in the matter. As Warwick, I might run down the Defense tree if I plan to try to outsustain the other guy and transition into a tanky initiator, or I could run a more Offensive build if I plan to gank and go all in for the kill.