[freyr]

I think your list of features is spot on, but I don't think their mainstream popularity had much to do with artistic expression. I'd include one other big feature:

   - They are dead simple to save (or link to) and drop into your own page/blog/comment.

and claim that GIFs are just really easy to share and view. They've offered a better experience both for post and viewing a short soundless clip than could be provided by an embedded Youtube player.

[mbesto]

How about the fact that there is zero advertising. IMO this is the number one piece of friction for videos (mainly YT) and the users don't want them for especially quick moments (a la Vine).

[michaelmior]

There's nothing really stopping people hosting GIFs from putting in advertising. Although it's true that I haven't seen examples of this happening.

[snogglethorpe]

> They've offered a better experience both for post and viewing a short soundless clip than could be provided by an embedded Youtube player.

I'm not sure that's always true, as there some very noticeable problems with GIFs. Most significantly they're often huge, and must be entirely loaded to play properly, which can result in long delays before they can be viewed (usually with no obvious indication to the viewer) and massive resource usage (besides the problems related to size, animated images also usually don't benefit from the sort of acceleration and optimization that videos do). A page with a bunch of embedded GIFs (not at all uncommon), can completely kill a browser.

[There are other problems, of course, e.g. the complete lack of user control over playback, but the size of GIFs seems to be one of the worst.]