From what i can see:
Good things - you can't see the grid very much (which is what you are supposed to try to eliminate when making a tileset)
Bad things - too small amount of shades makes it look... unreal. If you add in maybe 1 more? (unless you are going for that style)
- the water (i assume that's what it is) is kinda jagged. Change that would be good (unless you are going for that style).
- no variation. Maybe add in either cracks in the bricks, or random objects, such as boxes and similar things.
That's my opinon. If you want to keep your current style, atleast add in some variation in the tileset, as it is very repetetive. Hope this helps