The tagging system was not introduced at wordpress.org until the last upgrade there. Now shortly thereafter it has been introduced here at wordpress.com following our upgrade.
I have both wordpress.com blogs and wp.org installs and I'm delighted that this new feature has been introduced.
It will surely put an end to the bloggers who game the wordpress.com tag pages by using the same huge block of inappropriate and irrelevant categories at the end of each post they publish.
The number of categories assigned to any post that are displayed on the wp.com tag pages is severely limited. This will encourage the use of relevant and meaningful category assignments and under-cut the "tag spamming gamers".
In fact, the features I requested long ago have all been introduced. I'm a happy camper, for sure, for sure. :)