Some ideas:
* Charge for >3 minutes
* Cap number of screencasts on the "free" tier (charge for >x screencasts)
* Watermark/intro is free, without is paid
* Hosted for a fee
As far as freemium, they'll definitely have to be very careful with that, because if you're too nice you lose money, and on the flip-side you piss people off lol. Sometimes at the end of the day it works to be a paid app (especially if you're getting free marketing from Apple features).
But beware, once you're not featured, your sales can go close to 0 a day.
Without a watermark, I have things I can use this app for. While using it, if I like it and want the premium features then I'll upgrade. With a watermark, I won't use this, and I won't ever upgrade.
There are some great ideas in the thread; watermarking is not one of them.
* Offer accelerated (OpenCL) encoding for paid tier only. Well, assuming encoding takes much time
* Expire free tier screencasts uploaded to your hosting service. Free tier expires after 48 hours, paid tier lasts forever
export to various formats (flash, mov, gif, html embed, etc)
hosted videos with access control and email invites
advanced editing functionality (captions, effects, audio) and design templates