I think a lot of it has to do with the shading. Older comics did most of their shading in the pencils and inks because the color was so expensive to print, leaving everything with only one tone in the coloring stage. Nowadays it's shifted, so more of the shading in the pencils is carried over in the coloring stage instead of the inks, but when you apply modern coloring to older comics, the color shading looks a little redundant.
As for Neil Adams, they only recolored a couple stories from the Flash backups in the Green Lantern/Green Arrow trades, so comparing the recolored stories with the other stories that kept their original colors does leave you feeling like something's missing. It's not the worst I've seen (compare the Brent Anderson's art with the colors in X-Men: God Love, Man Kills with the colors on his art in the first Astro City trade), but it could be so much better.