When talking about creating a scenario you got a guide in the official site, but among the points you get to read how important it is to have a clear shot of what you want to make the player feel, in other words have a clear shot of the target audience you are aiming at, for instance, if I want to make a plague about people starting to see things eye-burningly bright, I'd put somewhere an event where TTG stops getting aired. With the question on the title I want you to think about what you search for in a scenario and what makes you think that the experience was worth it and that you should recommend it to somebody else. This is very important as the most popular scenarios are determined by the number of upvotes minus the down ones. I'm going to answer this question myself, and I want an answer more complex than "fun" as we all are in search of fun, we don't get retribution for playing scenarios, and even in that case, lacking motivation can severely hamper your efficience, but the point here is what is for you the source of fun, because I'm pretty sure a lot of people have fun with memes spawned more randomly than Squall's Death creepypasta, splat one after the other PPG 2016-like, or overly-simplyfied scenarios that are hardly different than playing with cheats, they wouldn't be ever present among the most popular scenarios of almost every week and month if that wasn't the case, but I am not able to enjoy such stuff. And now for what concerns me, here's what I search in a scenario, in order of importance.

• Customization-not in the sense of simply how much text, traits and images got customized. Sure, that indicates that more time has been put in, but what I want is a customization of the gameplay aspect, because if a virus plague has unchanged text and images, but the traits act differently, events are removed or replaced and overall the plague feels different than getasmuchmutationaspossibleandwinmegabrutal then I say that the plague got customized well and has a reason to be played over the standard game. Vice versa there are plagues, such as the Catgirls scenarios that are basically a copy-paste of the base template with barely any difference, and surely not enough to set them apart from it, despite being full of custom descriptions
  
• Everything must have a purpose-this is partially correlated to a balance question but not entirely. Sure, a trait that overpowers the rest to the point of making it useless then the purpose of the overpowered traits is defeated. Viceversa, if the point of some traits is to add points to the genetic drift like Bacterial Resilience, they might even not be there, but they might if that leads to a different strategy, like in the neural worm and unlike br. Some balance issues can make scenarios unplayable, mainly the scenarios with "ships use new sterilizing technique" wich can read as well "you can't win" and a Ben Broke laugh echoes in your mind.
  
• In-world consistence-with these scenarios you can have a world, but when the world looks artificial and doesn't flow naturally, you can't put yourself in that world. In fact to create a consistent world you set its rules and let the events flow. Nowadays the workshop overflows with Pokemon Go, and I have yet to see somebody think that the government could just bomb infected zones with the emps or shut down Google Maps, since pg depends on it. From these, for instance, you can start making events and traits to counter them and stuff