According to the Q&A section of (Japanese) Nintendo DS browser page, via IGN, Opera's add-on will not support Flash, video, sound, or PDF. And, while users will be able to bookmark pages, images and other elements from those pages cannot be saved.
On the positive side, as previously noted, the DS browser will support full stylus control, including handwriting recognition for easy navigating. The software will also store previously entered data. And, proud owners of the DS redesign should be pleased to learn that Nintendo will release a browser cartridge tailored to fit flush inside DS Lite's GBA slot. Good lookin' out.