Because I was curious myself, according to TokBox's FAQ
<a href="http://www.tokbox.com/support/frequentlyaskedquestions#techSpecs" rel="nofollow">http://www.tokbox.com/support/frequentlyaskedquestions#techSpecs</a>:
End users will need the following:
...
A standard DSL line with a minimum upload and download speed of 160kb per call participant
So it looks like it's a stable ~160kb per person, so if you have six people, a DM and five players, it should come out to about ~800kb up and down per user. At 10 users total, as advertised for Roll20, looks like about 1.6mb up and down per user.
Your typical residential broadband in the US might not have a problem with these numbers, but if you're somewhere less overrun with broadband providers, I could see this being an issue.
Aside: Also, in terms of our occasional flash issues, it looks like OpenTok will migrate toward HTML5 as the video tag becomes more accessible in browsers.