So many tags will be proliferated, that filtering by tag is likely to become as much a firehose as is the site now. I don't really see that solving the problem of relevance, except as pertains to creation of dedicated groups which gain prominence over others, e.g. #bitcoin becomes prominent over #bitcoinfans, #bitcointalk, #bitcoinnews, etc..
I presume Reddit can limit the number of discussion sub-forms created because it is a top-down run site?
As I said on my post on Bitcoin forums:
It works under the same concept as IRC channels. While not completely unbounded or analog, the variety of IRC channel combinations you can create is extremely high, yet people congregate/conform/cooperate as to funnel into certain ones.
See my rebuttal there.