We added image and video preview features to reddit last night. To enable thumbnail previews adjust the "media" preference on your preferences page to show thumbnails. There is also a new option on user-created reddits for whether to show tumbnails by default. This is good for reddits that are predominantly links to media: pics and videos, for example.
Right now the default user preference is to have thumbnails disabled in all circumstances. In a week or so, after we've worked the kinks out of this feature, we'll change the default to respect the current reddit's preference.
Also, you can now watch submitted videos directly in reddit by clicking the "watch" link next to video submissions. Right now we only support Youtube and Google Video, but it's simple to add support for more, so that should improve in the future. Sounds like a good job for open source contributors...
Enjoy!
