As you pointed out, the quality of decks is important. Limiting scope allows for creation of quality decks. At the end of the day spacing is just one aspect of forgetting - strong encoding is the other and for that you need rich varied content. Creating quality content is hard work. We started off with a focus on algorithms and engines and then realized that even if we slap the very best Memory engine if you have crappy content - no one cares (and retention is marginal over simplistic flash cards). We invested a lot of work on content and varied encoding and it's paid off and we are now branching into different verticals.