I am also having problems getting an animated gif to work as my featured image. I’ve read all of the other forums about this topic, and believe that I’ve eliminated all of the problems already mentioned.
The gif is exactly the correct requested size (1000×288), and does not resize. It is only 82k, with 2 frames, so it isn’t too large memory-wise. Pngs of the same size display properly. I’ve tried both normal and interlaced gifs, in both cases the image displays as a narrow bar.