You can only favorite images, not tags.
However, you can sign up for the RSS feed on a particular tag, though you need to keep on top of it because I think only the 24 most recent are shown in the feed. I haven't used it in awhile.
Posted under General
bluishwolf said:
...Favorite tags appear based on the images you selected
Moreso populous generic tags appear in your favorites. It'd be nice if there was some sort of Bayesian-esque statistic used instead of straight counts.
I mean you would think if I had 100 of the 10,000 posts with Tag X in my favorites, it should mean less than if I had 80 of the 100 posts with Tag Y.
Currently, I think the top so many tags are excluded from favorite tags to try to mitigate this, but as counts inflate, it will tend to happen no matter how many you exclude.
There would be a problem with my solution at the bottom where if I have only 1 of 1 tagged post it jumps to the top of my list. So you'd have to exclude infrequent tags (say < 30 instances) or something like that.
I don't know how much extra work this would put on the servers, but you wouldn't think much, since the total count for each tag and the total count per user is already there.
Updated by Shinjidude
I like that idea! It would definitely increase the value of the favorite tags list.
The only issue that I can see with your system is that nobody would have Touhou as a favorite tag, ever, even if you had 100 images and each one was tagged with touhou. That doesn't really make sense to me. I don't know how to solve it so that those two competing ideas are balanced, though.