ArenaNet is just full of surprises this weekend. Now they're letting everyone in on the fun by having the core game of Guild Wars 2 be absolutely free.
The idea is that by allowing players to play the immense amounts of content, quests and do all sorts of other great things in the base game, they could very well become excited enough to want to spend money on the upcoming expansion. That and there's the potential for using the in-game trading hub that uses real money, the Black Lion Trading Company, to purchase in-game items. Not everyone can necessarily afford to buy every great game that comes out, so this broadens the reach. So while it's definitely a change in how ArenaNet is looking to make money from their creation, it isn't free-to-play per-se.
Regardless, Guild Wars 2 has a lot to offer for both groups and those that wish to go it alone. And now Heart of Thorns is coming very soon and will have player run raids. And not just simple battles, but huge and almost impossible to beat sections of the game that require a well coordinated group of people making a concerted effort. Raids will be a key element in Guild Wars 2: Heart of Thorns. And to practice and get used to the intricacies, of course you can now play the OG Guild Wars 2 for free.