This is probably a hallmark of any sufficiently large, complex and popular software. There will be completely divergent (or at least different enough to believe themselves so) subsets of users with different use requirements. Without careful negotiation this can lead to deadlock.
Objecting to a feature enhancement that has no negative effect on your own preferred usage is probably not the best use of anyone's time. Exceptions can and should be made for changes which might accidentally affect your core usage though, such as changing the default rendering.
That said, I imagine any change in vim is going to get enough exercise that bugs will be found fairly quickly.
As for this specific feature, I imagine it would be some text hinting similar to coloring, but it would be ignored in console use, leaving gvim to do different font rendering based on hints.
I don't object out of spite. I'm certainly not the best or most experienced programmer here but I have an idea of the depth and spread of the changes that the proposed enhancements would require. The usefulness of these enhancements must be weighted against their impact on the code base and (and that's very important in Vim's case) the philosophy of the project.
Incorporating all these ideas into a new project, on the other hand…
