I thought Apple made a mistake by not opening the launch up to all iOS devices, iPod Touch, and iPads. I don't see any reason that the watch couldn't be used with them, nor the Mac for that matter.
The reason being is that so many people who own Android phones, may also own an iPad or a Mac. SO it expands the ecosystem for Apple.
Given the difficulties they've had during the launch I feel like ultimately made the right decision. But I would hope by Gen 2 they open the ecosystem to at least the iPad, giving more people access to it.