Updated:
Also Redis's main selling point is it's extensive data structure/operations support. "Blazingly fast" really depends on what your workload is and what you're comparing it against.
A few other notable Riak features include a JS or Erlang MapReduce API and full-text search.
(Updated: typo)
Blazing fast: I mean compared to the other four.
In memory operations are fast in many databases. Redis's default configuration (vm-enabled no) just only does operations in memory (with an occasional sync to disk). That's terrible durability but fantastic performance. Most databases, including Redis, can be configured for either that sort of high-performance/low-durability or the opposite. It's just that their default settings/behaviors vary widely.