The other devices don't have some of those functions because Apple did not allow access to those APIs. It's an unfair playing field because of it, but hey it's their playing field. Even within its own apps ecosystem, they provide second rate APIs - take the camera for instance. You can get a pretty good live view using the built-in camera control - looks to be like 15fps. Then, try using ProCamera's Watch app and try the live view - it's like 2fps. That's just what Apple does, which is why I'm not all that fond of them compared to other companies, but hey they can do whatever they want. So, as for other devices:
Pebble: Receive messaging, phone call notification and rejection, control media, navigation through companion app
Band: Receive messaging, phone call notification and rejection
No way is Apple going to let anyone else use Siri, integrate with their Maps or Apple Pay, so Watch is the only choice. That's how they operate and it has been successful for them to limit consumer choice. For me, I like interoperability. Power to consumer and whatnot. Meanwhile, I sold the Pebble - there's no point in having that now. The Fall update should be nice.