The best features in MySQL, making it one of the most used databases globally, are-
1) Multi-threaded architecture for multiple CPU utilization and kernel threading
2) Use of triggers and crash recovery techniques
3) Indexed Conditional Pushdown
4) Integration with LAMP/WAMP
5) Easy setup with Memcached for increasing performance and scalability
Room for Improvement:
Few cons in MySQL are
1) Increased complexity in InnoDB
2) Slower performance when using InnoDB as compared to MyISAM for using simple queries
3) Only identical MyISAM tables can be merged.
4) No active directory integration as in SQL server
5) It does not support check constraints
MySQL is indeed the world's most popular open source database. It has been designed on the hyper threading infrastructure and supports multi-threaded architecture for using multiple CPU's. It works on index compression with B-Tree disk tables popularly known as MyISAM. Its efficiency depends on the implementation of SQL functions with optimization of library classes.