I'm also in Ed tech space so I guess I am biased...but essentially:
1. Comprehensive curriculum. If I am going to spend time on your service I want to be able to get a working knowledge of the subject. Far too many Ed tech projects are simply an "intro" to the subject. The first chapter should be the intro while the rest is actually learning how to apply the knowledge or skill. Quality is key here. You are better off having the best online cyber security course rather than a collection of 10 average courses.
2. Interesting content. The content needs to be interesting, entertaining or engaging. Video and interactive content is key her (codecademy and the math content on Khancademy nails this). Bonus points if your interactive content is custom written for the subject.
3. A way for the learner to self-evalute skills or knowledge. Great, I read your chapter and learned a lot but did I really master the concepts in the chapter? I won't know unless I am able to take some exercises and get some feedback on my work. You would be surprised how many online courses have none of this or very poorly designed.
4. Analytics on my performance. I need to know if I am mastering the content. Bonus points if the platform/course tells me how to improve or what to study next.
5. Discussions. I need some way to discuss with others about the subject. Make sure trolls and spam are kept far away.
6. Design needs to be logical and user friendly. I should be able to learn very quickly how to use the site. All advanced settings in a separate page (settings dashboard).
7. Mobile friendly (especially for high end devices).
8. No progress locking content.
9. Prefer free over paid. Freemium model is also good. For any pay to use features or content you better bring serious value to the table.
10. Prefer to give less personal information. Why exactly do you need my date of birth, home address, phone number, name of my first born etc. You get the point.
11. Multiple paths to learning the content - textbook style (blocks of text), interactive video, interactive text efc.