Frequently questions here about why uploaded animated GIFs are inserted as still frames, and the "fix" is to upload in intended size and use original. Thats fine for people who understand wordpress media sizing, but your average Jane/Joe does not get this nuance (for that matter, a default medium width of 300px is tiny IMHO)
The thing is WordPress is quite able to resize animated GIFs and preserve animations -- if the uploaded GIF is smaller than the medium size, WordPress generates a thumbnail version that is animated; if the original is larger than the medium size, it generates a static thumbnail.
I documented the difference
How about enabling resizing of animated GIFs to all the sizes? or at least to any size smaller than the original?