Macros and closeups like this must be tack sharp.......and I don't think anyone will argue that this shot is not sharp............
Sorry....couldn't resist.
Back to your photo. If your subject was just the cactus needles then maybe you got what you were shooting at. Unfortunately, my eyes keep going to the right to the big green cactus trunk and away from the needles. The large empty space in the center, oddly enough also seems to be a distraction.
What really sets off a small light-colored subject like a cactus needle is a contrasting background (a dark background in this case). The light color of that empty space is just not helping you out here.