But now, that Cassandra has SQL like query language, hadoop has SQL like hive and other NoSQLs implement semi-SQL interfaces, I believe a good definition is a database that is not one of
1. ACID
2. Strongly typed table schema.
Having a SQL-esque query language is very far from the implications of "doing SQL":
- Any external software that relies on SQL (not a similar language that looks like but it isn't) won't work.
- Any tool which expects SQL and expects SQL-based drivers (like JDBC, for instance) won't neither work.
- SQL is a huge standard with a significant number of features. This semi-SQL interfaces, at best, implement a tiny part of that used to be SQL 92, a standard by the Windows 3.1 era. So not a big deal if you compare with modern SQL implementations in databases like PostgreSQL.
So I think they are still very bound by the definition that they don't do SQL.
There really isn't any such thing as SQL/NoSQL any more.