I wanted a /count that is really useful (for me). So here are my suggestions:

* /count <block name> - counts the number of blocks of the same type in the specified region.
* /count - simply counts the number of blocks of all types in the specified region.
* /count -<block name> - (with "-" before the block name) counts all blocks in the specified region except the block specified.

They can also be combined to give more specific results.
e.g.
*/count dirt grass air - counts all dirt, grass and air in the region as one.
*/count -dirt grass air - counts all blocks in the region except dirt, grass and air.

Thanks for your kind attention :D