I have the following:
- in progress - a Yes/No column which I set to yes when I started reading, and No when done. It allows me to create a virtual library. Not sure it's as useful as I thought it was going to be. The virtual library allows me to quickly go the book I'm reading if I've left the main library sorted in some way that doesn't leave it near the top.
- type - this is a fixed list of things like "novel", "short story", "play" etc. i.e. it's about the form not genre.
- category - this is genre I guess. As you say sometimes I think tags would be a better way to do this as many books defy genres, cross them or occupy more than one.
- words - filled in by the count pages plugin
- pages - also filled in by the count pages plugin (based on an estimate of 2150 chars/page). Sometimes I overwrite this with the actual page count from a paper copy. TBH I don't pay much attention to it. I mainly look at the word count.
- price - in £
- read - Yes/No for whether I've read it or not
- date read - when I finished reading it (for the first time). Some of these dates for older books are estimates. If the date is 1st of the month, or 1st of Jan, it usually means I couldn't remember the exact date

- notes - I don't use this much but I added it with the idea I might want to add my own notes, separate from the comments field which I leave for a book description usually from the metadata plugins
Making this list reminded me that I have a "Koboshelf" column which used to get filled in by a plugin-board. I've kept the same library format for a long time but I have re-install Calibre a few times and the formula for this has been lost. I don't miss it to be honest. So I've deleted it just now
The issue of whether something is better as a tag is interesting. I have virtual libraries for read, unread, in progress and started-but-not-reading. These match my Goodreads exclusive shelves. The last of these is based on a tag rather than a column. I could've added another Y/N column but it would clutter up the interface. OTOH for e.g. read, it's much quicker to change the visible Y/N column than it would be to add/remove a tag.
I think you find out over time which columns you use and which you don't really. I could probably get rid of a few of these. OTOH it's no huge hardship to keep them. If I ever do want to see if I'm reading more short story collections than novels I can see that quite quickly.