No modes, views, triggers, prepared statements, stored procedures, query cache, data conversion inserts, ACL. Fewer data types. Less engines, less code.
The simplifications that Drizzle introduce may also make possible features that really matter. Like DDL transactions (maybe), or faster subqueries.
MySQL carved out a unique place in the database world by being small, fast, and feature-poor - which was just what many users needed. Once it started to grow in popularity, people called out for features supported by the big boys, like triggers and stored procedures. MySQL 5 introduced a lot of these, bringing it to feature-parity with the heavy-weight SQL databases.
But now that the trend is toward document-oriented databases, these same heavy-weight SQL features are eschewed, even scorned. So Drizzle may be a MySQL that has come full circle.