Dimdim, like WebEx, seems to be optimized for presentations. For instance, they only allow 1 or 2 people to share their webcams (I think they even pitch the 2 webcams as "2 people can present together...").

AlmostMeet is more for those intense collaborative discussions within a small group. So everyone's an equal participant, even with the webcam and desktop sharing. And then there's a whiteboard that's actually usable.

Because we use P2P, the voice chat should also be better. The caveat with P2P is that it can't scale to a large number of participants, which is okay, coz neither does our interface :).