I don't discriminate against anyone based on their avatars, but I can't stand excessively animated, long, or flashy .gif avatars. The focus should be on the post; animated avatars take the attention away from the post itself and instead put it on the content's author. It's not unlike how some writers have their names written larger than the name of the story on their book covers. Their names are moving units, not their content, and I don't think it should be that way - especially not on a message board.
[small]Addendum: Unobtrusive gifs, such as dark colors, small size, or only slight animations don't apply. If it's not prominent in one's peripheral vision while reading, it's not an issue.[/small]