In the meantime, it'd be nice to know what moderators and developers think at this point - is this technically feasible?
It's a good idea and I'd love to have it (with some additional features), but I don't think it can be done well with SMF.
Thoughts on how best to do it:
Some tags (the current boards) would be listed as they are now on the front page. Additional less-popular tags would be shown in a categorized list elsewhere. A tag cloud could also be made available.
An "ignore topic" feature would be a good complement so you don't keep seeing the same topic you don't care about on multiple pages.
It'd be cool if users could define a number of "tag mashup" pages. Each page would list topics with any of several tags, and topics with certain tags could also be excluded from the page. Like a page containing topics with the tag "development" and "mining", but not "alternative clients" or "alternative currencies". If a topic matches multiple tags, it would only appear once on the page.
A separate report feature for incorrect tags would be nice. The reporter could add or remove tags from the current set for a topic, and moderators would see an easy summary like "+development, +mining, -discussion".