http://eune.leagueoflegends.com/news/inside-design-masteries-season-two
Variety is the Spice of Life

As they are currently designed, the masteries in League of Legends tend to support a small number of optimum paths depending on the champion role. With the redesign we want each champion to have a variety of choices depending on your particular playstyle. For example:

In the Offense tree, the new options are equally viable for champions that specialize in either physical or magical damage.

Defense has new options for initiators who want to increase their movement speed, or gain increased gold from kill assists.

The Utility tree provides new options for players playing support champions. For instance, they can use the new masteries to increase their passive gold gain and starting gold, so they can get the items they need faster.

In addition to supporting a greater variety of champions in each tree, we’ll also be increasing the number of options available at each level of the tree. No matter how you choose to specialize, every role has new options within masteries for improving their effectiveness.

Equivalent Exchange

We also wanted to normalize the value of each individual mastery point across the mastery trees. As we have added more items to the game, the value of altering certain stats has changed. With the cost-benefit relationship of each stat more strongly determined through all of this iteration, it made sense to reevaluate the masteries with this new data in mind.

In the new system, every point matters. Each mastery point that you spend feels like a strong, impactful choice.

Spellbound

Finally, we wanted a better way for players to select the masteries associated with their chosen summoner spells. Presently, selecting a mastery aimed at augmenting a particular spell might involve specializing deeply in a specific tree, even if it’s primarily focused on stats you’re not interested in increasing. Currently, reducing the cooldown on Flash requires heavy point investment in the Utility tree, which isn’t optimal for some roles.

Under the new system, each tree will now feature a tier 1 mastery that will boost all of the summoner spells associated with it. This way, all summer spells can be amplified at low cost, opening up the field for new strategic combinations, and affording you the ability to still specialize in the tree most closely associated with your combat style. Additionally, by giving summoners the option to select these highly-impactful masteries early, new players now have compelling choices available to them as early as level 1.

More Options, More Impact

These new changes to the mastery trees will provide you with better customization options for your champions each time you take to the Field of Justice. Additionally, you should no longer feel that some masteries are simply more powerful than others or that you’re obliged to specialize in one tree or another in order to boost a particular summoner spell. Instead, you’re free to focus on new strategic options, and new combinations that improve your combat effectiveness.