Currency can make or break a Free to Play title. If everything is gated behind the "real money" currency, you're going to be paying out your ass to afford what would have otherwise been a simple $15 subscription fee. Likewise, if everything behind the "free money" currency (which earned through constant play) is super expensive, the developers can drip feed you basic content with a cap while you potentially become addicted and eventually spend "real money" (or worse, limit your play to force you to spend money to keep playing).
Thankfully, our friends at Siliconera have unearthed the three types of currency available in Phantasy Star Online 2, so that we can get a better perspective of Sega's latest online action RPG -- meseta, arcs cash, and "FUN". Arcs cash is the "real money" currency of PSO2, which allows players to buy typical F2P MMO perks like extra character slots and customization features. Sega has made it known that like Guild Wars 2, it isn't in favor of giving anyone major advantages with Arcs cash items. FUN is a fairly unique beast in itself.
FUN is earned by doing meta-gaming actions like logging in a lot, and "liking" other players inside the game's contained social network. Funnily enough, you can also acquire it automatically by spending arcs cash (which will no doubt entice big spenders). With FUN, you can buy unique items, like stuff for your instanced house and for your character. As for meseta, like other PSO games, it's still the standard "gold" like you'd find in any MMO or RPG. As long as Sega sticks to their word, and arcs cash isn't used for anything major, I think Phantasy Star Online 2 will be a wonderful free to play dungeon crawler.