To steal an idea from Bungie, when it comes to games you want "thirty seconds of fun" -- a base gameplay loop (in the case of Halo, where it originally applied,

To steal an idea from Bungie, when it comes to games you want "thirty seconds of fun" -- a base gameplay loop (in the case of Halo, where it originally applied, it's the loop of taking out baddies from a distance, then the stronger baddies from mid-range, then mopping up the scattered weak baddies). An early demo (like the aforementioned Rust Legacy) is a proof-of-concept of that base gameplay loop, and if players like it, they can fund it. Once it's funded, they can build out from that base loop, and eventually you'll get a finished product.