Disclaimer: I've never actually run Dynomite, so I'm going on what I can find in docs.
In any case, a few of differences I found:
1. Configuration of N/R/W parameters. Dynomite appears to set these properties once, at startup time, for all values. Riak configures N per-bucket ("bucket" being like a 1-level directory structure), and R/W per-request. Specifying N/R/W so "late" means that applications can put different CAP requirements on different sets of data and tune those requirements at request time.
2. Dynomite's main external interface seems to be thrift-based, with also some RPC exposure. Riak's main external interface is JSON-based, over ReSTful HTTP.
