Preview through I think 10.3 supported playing animated GIFs. This was completely removed in the Tiger version--there is no play button availabe in the toolbar, and no "Play" command anywhere in the menus, either.
There was a brief discussion here about the fact that Apple didn't quite update the help file completely, so it contains a misleading phrase, but this is the important part:
"An animated GIF is a graphic file that shows a short animation. Preview lets you look at each frame in the animation as an individual still image."
Note the lack of anything about watching it moving.
I'm really not sure why Apple removed this from Preview, but regardless QT Player handles animated GIFs just fine, and with way more flexibility (again, scrubbing, etc) than Preview ever did.