For us, the main frustration was showing your screen to someone who you didn't frequently collaborate with. If they didn't have WebEx installed, you would have to ask them to download and install it, create an account, and then get invited to a meeting. The whole process can take more than 10 minutes which is more than enough for most people to decide that it's not worth the trouble, especially if it's for something like a sales demo.

We want to make it as easy to share your screen with someone as it is to instant message them. If we do, we hope people will start using it frequently throughout the day for all sorts of ad-hoc collaboration.