I think the question is relevant, I think other answers were very relevant and this was my humble addition to the thread:
I think answers above very very good, in my POV, the right NoSQL database for you is the one best fit your requirements in:
- Data representation: as said above, key-value, document, graph, etc.
- Data usage pattern: OLTP (high concurrency throughput, many queries and updates) vs. Analytics? (low concurrency, few big queries, no updates)
- Data availability and consistency: this is the main topic I wish to add
While all relational databases provide the virtues of ACID to keep transactions and data Atomicity, Consistency, Isolation, Durability - few NoSQLs provide full ACID, most do not provide full ACID but rather provide interesting tradeoffs around CAP theorem (http://en.wikipedia.org/wiki/CAP_theorem). Since you can't have all 3, different databases give different combinations, for example 2 NoSQLs from Apache, HBase provides CP and Cassandra provides AP (http://wiki.apache.org/cassandra/ArchitectureOverview).
Hope that helped.