The tiles themselves are good, but need contrast and variations in color. Some of them lack variety outside of colors, or are too similar in appearance/design. Take bricks and planks for example. Even the cobblestone seems like a scaled up version of those two, and the stone tile looks like it could serve as cobblestone (when compared to the default pack). These changes in stylistic direction may turn out confusing to the user.
Dusty's texture pack could be a good point of reference. There are some others I think you might find useful, I.E. Jolicraft or Doku's RPG (there's a 16x version somewhere as well). Unfortunately, good ones are hard to find. (But with our help you might land yourself a spot with them!)
Upon closer inspection these seem to be heavily anti-aliased
, using a lot of extra colors. I find it hard to believe all those artifacts were deliberately crafted by hand. If they were, however, there are quite a few (colors) you could cut down on.
Edit?