unable to start mysql 5.5.18 after new install... (no replies)

I installed 5.5.18 successful
drwxr-xr-x 2 mysql mysql 4.0K Jan 12 15:14 logs
drwxr-xr-x 2 mysql mysql 4.0K Jan 12 16:22 test
drwx------ 2 mysql mysql 4.0K Jan 12 16:22 performance_schema
-rw-rw---- 1 mysql mysql 38 Jan 12 16:22 mysql-bin.index
-rw-rw---- 1 mysql mysql 1006K Jan 12 16:22 mysql-bin.000002
-rw-rw---- 1 mysql mysql 30K Jan 12 16:22 mysql-bin.000001
drwx--x--x 2 mysql mysql 4.0K Jan 12 16:22 mysql
-rw-r--r-- 1 mysql mysql 113 Jan 12 16:22 RPM_UPGRADE_MARKER-LAST
-rw-r--r-- 1 root root 113 Jan 12 16:22 RPM_UPGRADE_HISTORY

but the DB will not start !!!

I took the my.cnf file from another running 5.5 db
and I get start up errors in the err log
I comment out one by one or error variables in the my.cnf file and tried start up
but I get stuck on unknown variable 'plugin-dir=/usr/lib/mysql/plugin'

the plugin dir is there

SO WHY DO I GET ALL THIS UNKNOWN VARIABLES ?
\
[note there used to be an 5.1 db there before
but I rpm -e uninstalled all rpm and deleted all data and log dirs..]

grep ERROR mysqlhostdb.err
120112 16:24:53 [ERROR] /usr/bin/mysqld: unknown variable 'table_open_cache=2048'
120112 16:26:58 [ERROR] /usr/bin/mysqld: unknown variable 'slow_query_log=1'
120112 16:27:20 [ERROR] /usr/bin/mysqld: unknown variable 'slow_query_log_file=/mysql/55/logs/ydb1_slow_query.log'
120112 16:27:41 [ERROR] /usr/bin/mysqld: unknown variable 'general_log=1'
120112 16:28:07 [ERROR] /usr/bin/mysqld: unknown variable 'innodb_write_io_threads=8'
120112 16:29:01 [ERROR] /usr/bin/mysqld: unknown variable 'innodb_read_io_threads=8'
120112 16:30:02 [ERROR] /usr/bin/mysqld: unknown variable 'plugin-dir=/usr/lib/mysql/plugin'


ls -atlhr /usr/lib/mysql/plugin
total 836K
-rwxr-xr-x 1 root root 227 Nov 13 15:43 daemon_example.ini
-rwxr-xr-x 1 root root 11K Nov 13 16:15 qa_auth_server.so
-rwxr-xr-x 1 root root 20K Nov 13 16:15 qa_auth_interface.so
-rwxr-xr-x 1 root root 15K Nov 13 16:15 qa_auth_client.so
-rwxr-xr-x 1 root root 19K Nov 13 16:15 auth_test_plugin.so
-rwxr-xr-x 1 root root 11K Nov 13 16:15 auth_socket.so
-rwxr-xr-x 1 root root 21K Nov 13 16:15 auth.so
etc....