h264 videos are generally significantly smaller than comparable resolution animated GIFs, not to mention being much better quality: It is a very good thing that the era of animated GIFs is drawing to a close, and the pace with which MP4 is supplanting animated GIFs is quickly accelerating.

Having said that, an h264 source can have magnitudes more complexity for playback, and generally the greater the compression the higher the playback complexity. Having five or so high profile multi-MB decorative videos autoplaying on a page is excessive.

They shouldn't use autoplay. They shouldn't even initialize until scrolled into view. They shouldn't all play at once. Their bandwidth usage is going to be enormous from this HNing, a single page impression pushing 15MB+ (EDIT: I hadn't really delved into it at the time given the display issues, however I grossly underestimated when I said 15MB).

Apparently [1] MP4s (at least their file sizes) can be optimized much more than animated GIFs, although it doesn't seem like that's been fully done here.

[1] http://gfycat.com/about