An actual recommendation engine would be a lot of fun to build. It would be awesome to recommend products based on subreddit clusters, and similarities between users.
But I don't really want to build a giant AI that sifts through the entire Amazon catalog. I feel that it's a nicer user experience when you just offer a small selection of manually curated products, and some studies show that limiting the number of choices can make people more likely to buy something [1]. I used to list a few CDs and MP3 downloads, but deleted those after I decided that there are already much better music recommendation engines, and they're subscription based. My line of thinking is that I should just start doing a few categories really well, and then expand later.
[1]: http://blog.ted.com/2012/07/18/does-having-choice-make-us-ha...