we spoke with a YC alum that has open sourced 90% of his codebase and recommended keeping a small subset of it proprietary. He said that when he was out fundraising it defused a lot of tension when VCs and angels would ask about his open source policy and he could say "well, I keep some of my hardest algorithms in a closed repo." As a game, we have the art, which I agree is a pretty substantial barrier to copying us (many tens of thousands of dollars in our case), but would you still recommend 100% open source vs 90 or 95%?Well, you have users beating down your door. Fundraising is never easy, but you'll have a much easier time with it due to your popularity. And if Sequoia are smart, they will invest in you, because you've discovered an entirely new game design. Historically, whenever someone has invented a game design that turns out to be immensely popular (Pong, Doom, Minecraft, etc), that's almost always resulted in the company becoming a hugely successful game studio. So if you focus on growing your community and on keeping the game fun, then it's quite plausible your studio will be hugely successful too.
Additionally, if you release your code 100% open source, you will get an insane amount of goodwill from the gamedev community. Right now the community is starved for examples of open-source games that actually work. Right now newbie programmers can go find some code snippets for various little tasks, little parts of a game's engine, like how to draw a sprite. But a tangible, cohesive product? Something that newbie programmers can git clone and actually run, and which actually functions? That's almost unheard of. (Quake and Doom engines are open source, but they don't provide any art assets, so newbie programmers can't download the code and do anything useful with it.) And that's a very good position to be in, because it's unheard of in the same way Rails was unheard of before it became popular. Meaning, if you do it, then you'll earn the instant loyalty of thousands of indie gamedevs. The wonderful PR you'll receive will be such a huge benefit to your company that it seems totally unjustified for anyone to be concerned that someone will steal your idea by stealing your code. (For what it's worth, I know of exactly zero times it's happened in practice. Code theft is a very overblown concern, especially in a hits-based industry like game development.)
You'll also enjoy many intangible benefits derived from open sourcing a fully functional game. For example, after Rails became popular, 37signals probably found it much easier to find and hire talent, since they were a sexy company to work for. I would bet you'll wind up in the same position. In fact, just thinking about you open sourcing this game makes me want to go work for you. If newbie gamedevs can download it, run it, learn how it works, tweak it, and become inspired to make thier own games... well, that's what gamedev is all about. It's why kids want to be gamedevs rather than other kinds of devs. So you'll be a huge boon to the gamedev community, which means talented people will want to work for you because of it.