The main features that I get out of HN are that (1) I get to read the opinions of a lot of very smart people and (2) I get exposed to industry news and trends that I wouldn't from sites with far more noise.
If I go to a site like http://answers.onstartups.com, I have trouble scanning the page because of all the visual elements like the tags, the background image, the blue text, the side bar advertising, and so forth. I realize not everyone is as visually distracted by those things as I am, but I really appreciate not having any of those page elements on HN.
The tags wouldn't need to be visible. Tag visibility is just a long-standing trend--they don't really need to be seen. An HN tagging system would probably only reveal itself when you wanted it, either in specific contexts or by enabling the display.
If I go to a site like http://answers.onstartups.com, I have trouble scanning the page because of all the visual elements like the tags, the background image, the blue text, the side bar advertising, and so forth. I realize not everyone is as visually distracted by those things as I am, but I really appreciate not having any of those page elements on HN.