According to the Dota 2 website, Valve tailored the current viewing experience specifically for the game with custom elements you might not see for other events. The company has added Live DVR features that make rewind available as soon as you load the platform, and if you don't want to chat in the general chat room, you can create custom channels for you and your friends.
The website clearly still needs a lot of work before it can become a bona fide Twitch rival. It still doesn't have a lot of features to offer and still has no way to broadcast your gaming sessions. If Valve chooses to keep developing it, though, then what we're seeing is but a small glimpse of its future.