I guess it is because, as the Natural Wonder is the cliff, it just looks like two grassland hills in most of it size. Nothing that suggests you should remove it to build an improvement.
Despite the additional appeal, they are still underwhelming, both benefit-wise, and appearance-wise.
I'm fine with most changes, but Work Ethic now seems broken if combined with the right panteons (to be checked if cards impact as well). I'd add it loses a bit of flavour by not tieing it to population (I mean, work ethic means your population is hard-working, isn't it). Maybe substituting the raw bonus by a +x production for each +y population would have been more flavorful.
Yeah, I think Cliffs of Dover would work better as two coast tiles instead of two grassland tiles. I wonder if that's possible, though. Can you attach cliffs to coast, or do they attach to the land tiles? They look like they're attached to the land tiles.