Does anyone have experience with building a concrete shower base and curb such that the concrete is the finish surface (ie, no tiling)? If so, are there any adjustments needed to the mortar mix to get a durable (and nice looking) surface?

I don't know how you would properly waterproof a surface like that, unless you did it like a conventional shower with a preslope and liner, then used concrete as the final slope with no tile over it. As for the curb, it would be difficult to make out of concrete only, given the aggregate within the mix.

As far as finishing it to make it look nice, I would personally have to call a concrete finisher to do it. Not one of my strong points.

That is exactly the idea--build it in the conventional way with a liner and a framed curb with lath--just skip the tile after mudding. My question is whether the conventional mortar mix would stand up to use or whether one would have to modify it in some way.