I think their explanation is pretty good:
It's not NASA saying the features "don't exist", as you claim -- it's NASA saying the features are most plausibly explained as airbag fibers, rather than as life.
I don't see what "similar thread-like features" you're referring to in that same picture. In
http://marsrovers.jpl.nasa.gov/gallery/ ... 53M2M1.JPG there is one long white thread-like feature just below the large rock in the upper right. What you presented was cropped from the same picture, about 3 thread-lengths to the left of the thread. Your image has a few black segments that look sort of thread-like, but are not nearly as well defined as the white thread in the main picture. Your image (from left to right) is about the same size as the large white thread, meaning the little black things AceCombat and I pointed out are no more than 1/3 the length of the big white thread, and most are closer to 1/10 that size. Furthermore, the white thing appears to be uniform thickness with smooth tapering near the ends, while the black things appear rough and poorly defined. Finally, in the larger image, if I point to the white thread, nobody will deny that there's something there (though they might try to explain it away as a "scratch" or something) -- it's a clear feature of the image requring explanation. If I point to the black things (in your cropped image, or in the larger image) it's very questionable as to whether there's even a feature there to bother explaining. Notice AceCombat and I didn't even identify the same things as "threads" -- but in the larger image, pretty much anyone would identify the white thing.
So, I agree with NASA: the scientists really *didn't* find any more "similar long, thin features" -- the things Ace and I pointed out aren't similar. They're not the right color, they're not smooth or uniform, they're not clear, they're nowhere near the same size, and overall they're just not the same thing. They investigated the one single feature (the white thread) and found that airbag fibers look exactly like that, and concluded that since an airbag hit that area, it's likely the thing photographed was a fiber.
You want very badly for the features to signify life -- like meathead in an evolution thread, you're just not interested in any explanation that doesn't fit with your preconceptions. It's rather unfortunate -- because establishing the existance of life on mars isn't something to be taken lightly, or to be done with shaky quote-unquote evidence that's easily explained. You're trying to establish an extraordinary claim -- that there's life on Mars. Such a claim should not even be taken seriously without reasonable evidence.