Unlike Mongo, which is a pin compatible architectural replacement for MySQL in most web app designs, you would probably never build a "to-do app" in Riak. Mongo is thus probably inherently easier to demo.
Moreover, if you just show someone the API for Riak and what it's capable of doing, you're not really giving them an appreciation of what Riak is about. To an application developer, Mongo's interface is probably much more congenial and full-featured.
I think there's a certain extent to which you either have the problem Riak solves and know why you'd want it, or you don't and no tutorial is going to change that. But that might just be an opportunity for a really creative tutorial.
Riak's documentation is horrible. It reads like a whitepaper; spending more time explaining how Riak is built than explaining how to build something with Riak. It's suitable when all you care about are early adopters, but will put off everyone else.
Trying to understand Riak by its documentation is like trying to understand Redis by reading up on ziplists.
Edit:
I'm the author of the two Little books mentioned above. I have no doubt that Riak's documentation can be much more approachable. (and I have a love-hate relationship with writing and would never write for money, so you can't hire me, if you were so inclined)
Can I beg you to write a little book on Riak then? I'm not a basho employee, I just hate their documentation every bit as much as you do for the same reasons.
Indeed. I'm glad to know that. Mind you, nothing in the home page or in the first pages of the wiki to give such an impression. When you read them, you get the feeling that Riak is yet another NOSQL database (which is absolutely fine - Mongo and Couch and Redis and tens of other NOSQL dbs all solve slightly different problems and have different characteristics and use different parts of CAP), so it's frustrating no to be represented with a simple tutorial and instead, being lectured on how Riak is built (which is slightly different that "how would you use it").
Now that I know Riak is a different NOSQL db, next time I'll spend more time reading the docs.
This is a good comment. I first got excited about Riak because I'm an Erlang programmer (hence why I knew about it) and tried to build a side-project web app using it; only to realize Riak is poorly suited for the type of task where you need anything remotely resembling the relational model (and all of the tools that come with it).
It hasn't been until two years later that I've actually found a need for it - it handles very specific pieces of our data model that grow rapidly and doesn't need to be queried or indexed in any complex manner. We still use a relational database for user records, profile records, transaction records, etc...
Moreover, if you just show someone the API for Riak and what it's capable of doing, you're not really giving them an appreciation of what Riak is about. To an application developer, Mongo's interface is probably much more congenial and full-featured.
I think there's a certain extent to which you either have the problem Riak solves and know why you'd want it, or you don't and no tutorial is going to change that. But that might just be an opportunity for a really creative tutorial.