I think the "oval stone surrounded by diamonds" has been around for ages (at least several hundred years) and it is a very classic setting. Actually most of the classic jewellery settings involve a stone of certain shape (oval, pear, marquise, round, square etc.) surrounded by either one row or several rows of diamonds. I'd think in the 1980s people liked overly large costume jewellery and weird modern designs.
I don't like the mixing of white gold and yellow gold and I do think the setting could be more delicate (which is why Kate's jewellery probably looks dated) but other than that, I don't know what kind of jewellery designs would be considered beautiful these days.
Oh, and those pearls are butt-ugly!
I wonder if they're real. She seems to like them a lot though for whatever reason.