Mongo was not designed with horizontal scaling in mind. Riak, Cassandra, HBase, Project Voldemort...these are the projects that were designed with horizontal scaling in mind (as evidenced by their architectures.) But not Mongo.
There is a large set of nice features that makes Mongo, for most people, nice to use long before you even need to address sharding. The percentage of people that will need to shard is much lower than the percentage of people that can get considerable benefits of how you can query data in MongoDB, for example, vs. Riak.
You are such a Riak-lover. :P