OpenDJ groups management
Did you know that OpenDJ adds nice features to optimize user groups management ? For example, one can define virtual static groups, to present dynamic groups as if they were static.
Also, you can finely tune the directory entry cache, to maximize search or modify requests targeting a group: multiple caches can be defined, with different priority levels, different cached entries (bases on a LDAP filter), different sizes and purge trigger thresholds, as well as different cache strategies (FIFO,LRU).
Not so many directory server, if any, proposes such flexibility.