MIcaiah, thanks for the detailed and well thought-out response. I'll try and respond to some of your thoughts:

I imagine this would be very hard to monetize and get customers as-is.

As far as monetization goes, I think the best route would be to charge online education providers on per-API-call basis. The end goal would be to become something akin to the "Twilio of online learning." With a sufficiently developed system, I think it'll be possible to convince companies in the online learning space that this is a worthwhile value proposition for their users. End users who have committed to a particular online learning program are much more likely to be willing to use a spaced-repetition learning system to aid in their progress in a particular course.

Your emphasis though, would appear to be more oriented toward existing power users of SRS.

I think I gave the wrong impression here, I think I'd much rather target non-users of SRS. Building something simpler but more accessible seems like a more viable alternative. Gaining traction with average, non-SRSing users, and then later adding best-of-breed features to tempt online learning providers seems like a more reasonable approach.

The average user needs to be sold on the effectiveness of a product very fast, on the first usage (or perhaps even sooner!) in order for them to continue using. However, SRS software in general are almost by definition antithetical to that goal: Their benefits do not come until far into the future, worse still it's an undefined time in the future.

I've thought of a couple of simple use cases for this sort of platform that I think seem easy to build and quite compelling for an average non-SRS user:

• Vocabulary expansion - For People looking to expand their vocabulary - a simple javascript bookmarklet that would allow users to learn the definitions of new words that they come across.
• "Remember what you read" - It seems that given the number of things that an average person might hope to learn in a particular day, but which are instead soon forgotten, having a simple way to record those items would be quite immediately valuable. For example, as soon as I found Instapaper, I began saving documents that I wanted to read later. I could see a simple javascript bookmarklet for "things I've read online and want to commit to memory" being used in a similar fashion. This implementation would be a very, very crude version of SRS, but I think it could help get users on board.
• Name-Face Identification - A tool that helps users learn the names of all the contacts in their LinkedIn or Facebook friends lists. Forgetting the names of acquaintances is a common problem, and an SRS program is an ideal solution.

Beginning with a simple, self-curated deck like the ones described above would also help to avoid the problem of not having good content for first-time users.

Very interested to hear feedback on the above.