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.
I have to respectfully disagree. Your comment is a bit sweeping and I really don't think that MongoDB's sharding solution is a bad one ... simply the strategies are different.
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.
Mongo's sort of a weird halfway point between "totally horizontal" stores like Riak or Cassandra, and "Hope you have a ton of RAM" stores like your traditional RDBMS setup. I think it's hard to look at it and say that it wasn't designed with horizontal scaling in mind, but it's totally fair to say that it wasn't designed purely for horizontal scaling, either.
