Isolation levels
The contract for which concurrency anomalies a transaction may observe, traded against how much concurrency the engine allows.
The defaults differ between engines - PostgreSQL and SQL Server default to read committed, MySQL and MariaDB to repeatable read - so identical application code behaves differently on each. Assuming portability here is a common source of subtle bugs.
Explained in depth
PostgreSQL internals that decide production behaviour - MVCC and XID wraparound, autovacuum thresholds, work_mem, WAL durability, indexes, replication.
SQL ServerSQL Server edition limits that decide architecture - memory and core caps, Always On versus FCI, RCSI and tempdb, Query Store, core-based licensing.
MySQLMySQL in production - InnoDB buffer pool, redo and undo, REPEATABLE READ and gap locks, GTIDs and Group Replication, and the 8.4 default changes.