Why did you want to make games?
[... P]eople are spending more money, time and attention on playing computer games than ever before, and games are constantly evolving into a more nuanced medium of expression and a greater part of our culture. We think this culture should allow gamers to learn how their games work, change them, share them and redistribute their works. This freedom to tinker is what Free, Open-Source Software (FOSS) is all about and so we hope a lot of people tinker with 0 A.D. [...]

What one thing would you tell someone to convince them to get your game?
0 A.D. is open-source, free software with cross-platform multiplayer capabilities. So you will always be able to download it from the internet for free, share it with your friends, and play with them online, even if they run Linux and you don't.