Now I know for certain that the beta only contains the first two acts. A commenter informed me that they will release Act 3 in a few months and Act 4 will be available at launch. The devs are also planning to release additional acts and content with time.
While the story feels thin to the point of nonexistence, the commenter also tells me that they are going to greatly expand on it, but for now, it's entirely acceptably the way it is, considering that the game is still in beta.
The currency system is also being worked on and the devs are planning a trading system where people can exchange items and currency without logging in, directly on the website. If that happens, it would be really neat.
Path of Exile felt really awesome. I took my Marauder through Act 2 and, while exploring the insides of a temple, I walked through a portal and found myself back to the the shore where I first started the game. For a second I wondered what was going on but they I realized that this was actually the next difficulty level called Cruel. I don't know how many difficulty levels there are. If my commenter sees this, I would be grateful if he could clear it up for me. I've seen talk in chat of something called Chaos so that might be the equivalent to Diablo's Hell. Either way, there's a lot of replay-ability in this game.
One other thing that I forgot to mention is that Path of Exile does feature randomly generated levels.
Just to give you an idea of progression, I finished Act 1 at around level 18 and Act 2 at level 30. My plan is to switch to a ranged class, most likely the Ranger and take her through both acts to get a feel for the class. I might also run her in parallel with the Witch.
I played a bit through Cruel difficulty and the monsters are a lot tougher than on Normal. While regular monsters in Normal could be dispatched by a Heavy Strike with my 2-handed axe, those in Cruel require 2-3 hits and they also hit harder in return. On the other hand, the item drops are slated to be of higher quality. I've also noticed a higher incidence of Champion and Rare monsters.
My presentation of Path of Exile's beta ends here, not before presenting you with some more random screenshots from both acts.
Act 1 Normal
Act 2 Normal
Act 1 Cruel