I think any subject should be open to discussion but that the netiquette rules should be paramount - so people can express their opinions of the matter in hand but they must not simply descend to invective or ad hominem insults. It is perfectly possible to debate/argue about anything with a little bit of detachment and it is usually fascinating, but if people can't debate without insulting each other then they shouldn't debate at all.
The difference is simple - if I say "I love marmite on toast ooh the savoury deliciousness of it" and someone else says "oh no marmite on toast makes me feel ill, I can't bear it, it is far too much like tar etc" and someone else says "try marmite on a slice of courgette" or "it must be terrible for your blood pressure with all that salt" and then someone else says "oh but the vit B in yeast think how good for you that is" & then they drift onto things like miso & vegemite then that is fine.
If, however, someone says "people who like marmite are obviously tastebudless halfwits, you are a tastebudless half-wit, in fact you are the most tastebud deprived and halfwitted person I have ever encountered" or "you like marmite on toast so you are the incarnation of evil" or a variation on that theme, then that is quite different.
We are all adults & should all be capable of being polite, whatever the topic, is what it boils down to. We aren't obliged to read a discussion or comment, after all, so if we choose to we should stick to the rules.