The first is that many other educators have told me that YouTube is blocked in their building/district. There are now many different video hosting sites out there that work as well as or better than YouTube and I'll bet these haven't been blocked yet.
Another reason I like Blip is that when you upload a video to Blip you can have Blip upload it to the Internet Archive at the same time. You will need an Internet Archive username and password for this. (for more on how to upload videos to the Internet Archive, check out one of my previous posts)
My other reasons include: there is no 10 min limit for videos as on YouTube, the original source file is made available for download, there is not as much cross linking and recommendations making it less likely that students will be led to inappropriate videos.
This is the time of year when my seniors are gone and my juniors taking physics do astronomy projects using Photo Story. This year I've been posting them to Blip.tv. You can check out the ones I've uploaded so far at falconphysics.blip.tv. Unfortunately, the files created with PhotoStory use a video codec that Blip doesn't recognize (ironically, YouTube can convert these files), so they can't be converted into the typical flash video format. However, you can still view the original video files in *.wmv.
I've embedded one of my students videos below, but since it wasn't converted into flash video it might not display correctly in your web browser. (Note: The student narrating this does not have a British accent. He just decided to adopt one for comedy value)