First of all, I mentioned colors. The new Castle line brings us a very nice varied and colorful palette, among which various shades of blue and brown. Yet these colors are not as overly bright as the primary colors used in the 80s and 90s, which adds a touch of realism for me. And lets not forget the metallic armour, helmets and weaponry, which in my opinion also look much more realistic than the old grey and black ones.
The prints have also come a long way since then, the designs are very detailed and of great quality. Though, I do have to admit that some of the torsos would be better off if they were less faction specific, since they would make great torsos for other factions (the archers scale mail comes to mind).
I think set design has also greatly improved over time. In part because new parts became available, but also because designers seem to experiment with different building techniques more nowadays. I doubt they would've come up with sets like the Troll Battle Wheel back then, even if the parts had been available. And it is exactly this originality and innovation that I find so refreshing in the current designs.
Hence my conclusion that the current Castle line surpasses previous ones at least in terms of aesthetics. Though this is ofcourse just my personal opinion, and I'm the first to admit that the Castle themes from the 80's and early 90s definitely had their own charm and appeal ;-)