2. Uncensored, Unique Theme
I strongly believe that game enthusiasts want what they haven't seen yet, and that adult gamers should be treated like adults. Some people might argue that Isaac isn't "mature" when it comes to its content, but those people would be ignorant fools! When I designed Isaac's story and overall theme, I went in wanting to talk to the player about religion in a manner I was comfortable with -- that is, with dark humor and satire.
A lot of the content in Isaac is extremely dark and adult. It touches on aspects of child abuse, gender identity, infanticide, neglect, suicide, abortion, and how religion might negatively affect a child, which are topics most games would avoid. I wanted to talk about them, and I wanted to talk about them in the way I was comfortable with, so that's what I did with Isaac.
I'm not saying everyone who played Isaac did so because they cared about these themes, or that they even understood why they were in the game, but I strongly believe that this adult conversation I dove into with Isaac is what made the game stand out to people and kept them thinking.
I grew up in a religious family. My mom's side is Catholic, and my dad's side is born-again Christians. The Catholic side had this very ritualistic belief system: My grandma could essentially cast spells of safe passage if we went on trips, for example, and we would light candles and pray for loved ones to find their way out of purgatory, and drink and eat the body and blood of our savior to be abolished of mortal sin.
As a child growing up with this, I honestly thought it was very neat -- very creative and inspiring. It's not hard to look at my work and see that most of the themes of violence actually come from my Catholic upbringing, and in a lot of ways I loved that aspect of our religion. Sadly, the other side of my family was a bit more harsh in their views on the Bible; I was many times told I was going to hell for playing Dungeons & Dragons and Magic: The Gathering (in fact, they took my MtG cards away from me), and generally condemned me for my sins.
I wanted Isaac to embody this duality I experienced with religion. I wanted it to show the positive and negative effects it had on me as a child -- the self-hate and isolation it instilled in me, but also the dark creativity it inspired. The Bible is a very good, creatively written book, and one of my favorite aspects of it is how so many people can find different meanings in one passage. I wanted Isaac to have this in its story as well, which is why the game's final ending(s) have many possible interpretations.
3. The Wrath of the Lamb expansion
Doing a DLC expansion was never in the plan for Isaac; I assumed the game wasn't going to do well, so it wasn't something we really ever even talked about. I had a few pages of "dream ideas" that I wanted to add to the game, but I had to stop working on them and put them on the back burner, since I wasn't sure how much the extra content would matter if the game didn't do well. Six months later, we ended up taking these dream ideas and expanding them into an extra-large DLC expansion.
The Wrath of the Lamb expansion added over 80 percent more content to an already-bloated experience -- and people ate it up. 25 percent of the people who purchased Isaac also paid for the expansion, and that ratio is going up by the day. We honestly didn't expect to make it, but once I started seeing such a positive, creative fan response, I felt obligated to continue Isaac's adventure.
Honestly, however, the number-one reason why I did the expansion was because my wife Danielle had already 100-percent-completed the game. It was the first game I had designed that she became obsessed with (she's actually playing it right now, behind me, while I'm writing this), and it made me extremely happy to see her fall in love with something I had made. I just had to continue it (also, she wouldn't shut up about wanting more).
4. Circumventing Censorship with Steam
Steam is amazing, and with the Isaac release experience I've found another crucial reason: You can use it to sell uncensored and unrated games. This was vital with Isaac, because I wasn't going to bother with getting an ESRB rating for a game I wasn't sure was going to sell more than 100 copies. Valve knew the game was weird and could possibly get some backlash, but they allowed it on Steam because they felt it had potential, and I love them for that.
Another huge plus to working on Steam was the ability to constantly update Isaac with fixes, updates, and new content. They would upload a new build within the day we submitted it to them, and if we had released it on any other platform this would have been impossible (and probably cost us about $40,000 to try).