Is MariaDB a suitable drop in replacement for MySQL on a standard LAMP stack?
One valid reason to move to another MySQL flavour is performance. The latest MariaDB versions 5.3 and 5.5 (which are drop in replacemants of MySQL 5.1 and 5.5 respectively) included major improvements in query optimization. You can read more on their site: What is MariaDB 5.3

Examples on improvements/features are "Index Condition Pushdown", "Cost-based choice of range vs. index_merge", "Subquery Materialization", "Subquery Cache" and new joins methods, like "Hash Join". Several of the features affect performance of (statements with) subqueries, derived tables and views.

Details can be found at the Optimizer Feature Comparison Matrix page.

Other additional features like microsecond support for time datatypes and improvements on the replication and binary logging.



The previous versions (5.1 and 5.2) also have some features that are not found in (mainstream) MySQL, like [PERSISTENT] VIRTUAL columns.



Of course, any migration of your server from MySQL to MariaDB should be examined and tested by you as whether these features are useful in your databases/applications and whether the difference in performance (if any) is big enough for the troubles you may have (as with any migration).