The dependency on Flash will shape the market segment that is likely to participate. Overall, the linear nature of video means more substantial time commitments than a text based format: skimming video is hard, amateur webcam video of arguments is a use case for skimming, and people who produce high quality video probably have other options.
Maybe a Twitter like approach...20, 30, 40 seconds limit might work.
I agree that the main challenge for us is to get substantial time commitments from the users to create a quality video and post. Most people are not that vocal, but some are - specifically politicians and celebrities.
Twitter started with 140 characters and eventually had to remove the restriction - so not sure if limiting the video length would be helpful. Alternatively, I think 20 sec preview/trailer of the video would be useful in filtering out uninteresting videos.
I see some merit in doing video only based debates & discussions - reduces the chances of abuse & troll via anonymous and coward text comments. More often, useful comments are lost in sea of useless and irrelevant comments.
If two people are going to go back and forth on video, why not use YouTube or Google+ or Facebook?
Why not serve HTML video to everyone?
I wonder if scanning video for problematic content will be easier than text, e.g. 1/2 seconds of sexual content sprinkled in a five minute video may require watching the entire thing.
Youtube used to have video responses which they discontinued due to lack of engagement - however, Youtube is a very generic video site and not all types of videos are meant to have video responses.
And if you give an option of text comment, most people may tend to use it first because they are familiar with it.
Existing social networks such as Youtube, Google+ or facebook are not conceptually designed for video debates & discussions - i.e showing all video responses of that specific topic in one place. "Related Videos" section does help a bit, but not fully.
Today, people are extensively using facebook to posts video selfies where they talk on a topic or express an opinion, but the responses are limited to text comments and it does not give the feel of a proper debate like people do in real life.
I've provided a feature of tagging a video response as "for", "against" or "not applicable" - thus giving it a Debate like feel.
Scanning for video content is a challenge - currently planning to do it manually till it becomes a scale problem, and also depend on the users to flag inappropriate videos.
Maybe a Twitter like approach...20, 30, 40 seconds limit might work.
Good luck.