After seeing all three 'Summer Friday Syfy shows' end their seasons, Killjoys is the one that I'd both really want to see more of and that warrants another season. Defiance too had a great season, but the finale just worked great as a show finale and I can't quite see how it could well continue.
Killjoys' characters had great chemistry together. The Quad and its inhabitants were mostly made a believable society. The stories were 'fresh' in that they brought original elements to often tired old plots. And it was fun.
I very much hope it stays fun - if it gets another season.