I've played my 3DS maybe three hours total since launch day and I noticed my moves in SSFIV were not working. After a couple of minutes I finally realized the slide pad was broken (it wasn't obvious at first). It's very difficult to move in some directions, and other times, it's fine, but it goes right back to being frozen.
I guess it's a common issue, since it was one of the choices for the RMA to Nintendo on their site.
Anyone else have any issues with their 3DS?