W.r.t. catering to "I just want to play, not be bothered by the main quest" players, I think the main thing is just to ensure that ignoring the major quest-lines doesn't hinder progression in other areas. Say, in game A, the reward for completing main quest milestones is the unlocking of new continents, the availability of new crafting materials, new spells, new party characters, and experience points. In game B, the only main quest rewards are new party characters and experience points. If you want to cater to the "anti-mainers", then tend towards game B, so that they can pursue their own quest (say, become the greatest blacksmith) without ever having to say "Damn, I need some obsidian, and it's only on the other continent; I guess I have to go fetch six dragon eggs for that idiot king after all."
I'd put myself in the "anti-mainer" camp, actually. Not because I don't like stories, just because at this point I'm tired of "fantasy main quest" stories. I wouldn't say there's a ratio I'm looking for, though. It's more a structural question, of whether I can genuinely pursue my own "quest" independently of the main quest.
Actually, what I really want isn't sidequests, but progression that isn't tied to a quest structure at all. While playing Skyrim, one of the things that took the depth out of the world for me is that there were few details that weren't eventually part of your big list of quests. I kind of wished for... to use a school metaphor, "self-study" rather than "taking courses". I might have liked learning the "shout language", or collecting the singing plants, or doing "archaeology", except that in each case I couldn't quite feel like it was *my* goal. It all feels like it's just there to support some quest... and there's a feeling that if I'm not on that quest, I shouldn't do *anything* lest I mess up a future quest and not get fully rewarded. (Like, I shouldn't kill that particular monster, because killing it is almost certainly a future quest, because there's little non-quest-related material in the game, and so if I kill it too early I might miss out on an important reward.) In other words, making activities into formal quests and adding external rewards to them can lead to perverse incentives, inhibiting the player from pursuing that activity unless specifically instructed to.