Ultimately any task where you care more about high speed and low cost than you do about consistency is a great candidate for NoSQL. However, I have to comment on this one.
Users/groups and ACLs: ('To some degree, LDAP was the original NoSQL database').
There are vulnerabilities in AD that resulted from this philosophy. For example: http://social.technet.microsoft.com/Forums/en-US/exchangesvrclientslegacy/thread/3da53460-ef76-4f01-94c9-f7b96fdaf99d
I also find the high-frequency trading bit to be really scary. Given the number of news stories about these applications going rogue I'm not sure I'd use it as a poster child for NoSQL. Not that these issues are related to NoSQL.
-DW