I have nothing against tags! I not looking to replace them, I use them extensively in Lightroom and Endnote, and I plan on doing the same in Calibre.
The reason I want a subject field, is so that I can quickly see all of my Philosophy works, History works, Literature works, etc. My library will quickly have hundreds of tags, I'm sure, including lots of very specific info that would only apply to a few works. I'm sure there are other ways to accomplish what I want, thanks to Calibre's many customization options... I could make a user category of 'favorite tags' or something. But this way works fine too!
I am not a tag hater!