Hi Damien, you probably know my reply already since we discussed it off-line
For me Companion mode is the most important. I would like to receive notifications of who is calling (so I can keep my phone on mute most and out of sight), notifications of txt message and being able to view it on the smartwatch (typically an easy implementation since its a short plain unformatted text), notification of calendar events so I don't miss a meeting or an upcoming appointment, and email notifications with a preview of the message.
In my personal opinion, answering call from a phone is optional but not necessary. Phone call is a private thing. I just would like to see who is calling and then make a decision if I want to dismiss the call (maybe with a simple rejection button or an option to send a few pre-defined txt messages like "i'm busy" or "will call you back soon"). If I need to pick up the call, I can always get my phone out of the pocket or pick up the call on bluetooth headset. With text message and calendar events, as I mentioned above, it is nice to preview on the screen of the smartwatch where I think 1.54"+ display is big enough to preview simple short text. With emails, it would be nice to see who is it from and body of the text. I wouldn't expect replying back from the watch; if its important and needs my immediate attention - I will take out my phone and type away the reply.
Of course, music control (of the songs on the phone and also songs stored on internal to smartwatch microSD card) would be important. Also, getting weather notifications from the phone is a big plus, as well as battery status of your smartphone.