Now that tags don't explicitly matter, what is their purpose? To help with similar posts? To help me curate the content I see on the front page? Because it's human nature to categorize things? The questions that follow are highly dependent what one would answer to the above: Is one post only meant to have one tag? Two? A hundred? Should tags be ranked by users by their relevance? What if a troll decides to tag everything on the front page with #dicks? Should the original poster be able to see who tagged their posts with what? Restrict tags altogether? I'm rather fond of how what.cd implements tagging. Half to help the content browser quickly select what they think is interesting 1, 2]and half to construct pretty maps like this one. Having extensive tagging like this opens up the door of post suggestions, like Netflix does with movies, but I have no idea what the signal-to-noise ratio would be when you give the reigns to the users of the website.
Yes, my opinion is that now that following tags is out (and that's something I've thought would be good for quite a while), that we would be better served by allowing multiple tags on a post. The reason is that a singular tag functions as a community organizer, while multiple tags can transcend a singular community and hopefully bring groups together. I am lobbying to have a character limit in the tag field and nothing more. That limit would also be arbitrary, obviously, but the other option is infinity, and we have to stop somewhere. Some happy medium probably exists.
Keep in mind that keeping ignore around opens this up to (even accidental) abuse. If I ignore #atheism and someone posts a #cosmos video that another person finds areligiously inspiring, I won't see it, despite loving the spiraling shapes of galaxies! I'm tempted to suggest a probabilistic ignore, if the votes for a particular tag exceed 25%, say, keep it out of my feed (Balancing this with the number of users I follow suggesting the post to me, perhaps). But I also have a tendency to suggest overly complicated solutions, so pick and choose what you think is actually decent.
It's a discussion that b_b and I already started having. Not using tags as subscription actually creates some possibilities. I want to use them this way a bit and think on it. However, that what.cd implementation is interesting. Something like that was on the table when we came up with community tags. It's a possibility that tags will evolve as a result of this.