New gravel looks almost like how cobblestone should look, IMO. So gravel is now one of the ugliest blocks to me. (I changed it myself since, tho) Old gravel wasn't perfect - just needed to change the hues, mostly.
Obsidian is kind of ugly, but it's not that bad. Explained: the colours are about right, but it's a bit too dark/flat - it screws up depth perception when looking at large swaths of it or structures made with it. Yuck.
And I've never been much of a cobblestone fan. (Love mossy cobble though)