To an approximation, the entire user interface and everything about how it worked. Edit: A more wordy response, since I assume the downvote implies that wasn't

To an approximation, the entire user interface and everything about how it worked. Edit: A more wordy response, since I assume the downvote implies that wasn't satisfying -- It has all the key elements - video chat, screen sharing, whiteboarding, etc., but they've all been arranged in such a different way that it's just impossible to single anything out in particular. The best I could do is explain it by analogy: Most software is designed to work best as either an online equivalent to a conference room, or an online equivalent to a lecture hall. But, IRL, a classroom isn't quite like a conference room or a lecture hall. Depending on the type of classroom, the physical differences might be subtle, but they are pervasive.