nice article, though i'm not sure that's the One Thing. my one thing would be 'know your audience'. that's part message and part usability. if you're designing an app for engineers you design the ui very different than if you're designing it to allow residents of the local retirement community to self-schedule dinner deliveries.

that said:

if you're a resident in a company with a separate marketing team, it's key to speak their language -- after all, the people you're going to be marketing -to- is the marketing team. it's their job to get the message out, it's your job to be sure they know what the message is, and what it isn't. make sure you don't oversell, and let them know what would be overselling, and promote key features internally; they'll get external through the firm's marketing wing.

if you're a startup, marketing is doubly important. if you can't sell a friend on the idea of a product, you won't be able to sell the public. if your startup idea is complex, you're going to have to find a way to make it intelligible in ten seconds by picking the key features you want understood. and everyone in a startup is on the marketing team, whether they like it or not.

it's really worth it to read a book or two on marketing, if for no other reason than to get the lingo down. i've found my suggestions much better received when i could speak market-speak to the marketing team and sales team, and promote effectively to civilians. i recommend 'the culting of brands' by doug atkin as a good start.