Are there any apps out there for the Mac that allow for any interplay between the watch and a Mac? Seems a shame that they're part of the same ecosystem, yet they don't seem to be too connected.
None that I am aware of. You iMac might see the AW Bluetooth but would not know what to do with it with out drivers. The watch is made to mirror the iPhone.
What exactly do you want to be connected? There is no benefit to the two being connected. The watch won't gain anything from being connected to a Mac that it doesn't already get from being connected to a phone.
In the case of my mac mini, it's connected to my Pioneer AV receiver and is also my HTPC (rather than using an ATV) - it's enjoyable browsing on my 65" plasma.
I use the remote app on the watch to control my iTunes on the mini. (and, before anyone asks - the artwork is just a vague, distant coloured square in the background of the remote app - I wouldn't expect nor need it otherwise)
Oh...and I want Apple to do up the Activities app for the mac, so I can see nice graphs of my history on a decently sized screen (as opposed to iPhone).