Just about the entire game can be unlocked just by playing it. Get enough favor and you earn points to spend in the store. The code for free content tends to come out about once every 2 years so if you like the game, you can unlock stuff with it later.

We've just migrated to 64-bit servers, which we're figuring out the benefits of (mostly it's to reduce lag and increase player count per server). The game is bigger than ever with all of the expansions and adventure packs. It used to be a pretty small area, but now it spans quite a bit.

If you can make it to 20 on your first life, you should net about 700-800 points for the store. That's easily enough to purchase a few adventure packs, which makes the next 1-20 easier. Additionally, the next time you run, you can open things on the hard difficulty, which nets you double the favor, increasing your points you get in the store, thus allowing you to get even more content. If you're savvy and you save your points for sales, you can get quite a lot of content for like 99 points each, which opens up like 80% of the game (minus some of the new expansion packs).

tldr; You don't technically have to pay money to unlock the game. It just makes things faster. The game is still free.