>replying to ghosts from 2013
We would never know if OP is still lurking around or not. I'm here since 2011-ish, and I had some of my ancient threads bumped. I didn't mind at all.
>First person exploration is nice if well executed
For sure it is nice! I love first-person games (of any genre) for the very reason you mentioned: the feeling of immersion. Actually, I've been developing my own first-person FG for a couple of years now (I just didn't announce anything yet, as I hate generating hype before I have something solid to show).
>requires a lot of work as you have to add details to the zones the player can explore
If you are familiar with 3D world design, I don't think it would be much work. For example:
-You don't have to worry about lighting and shading, your engine/lightmapper will do it for you, and it will always be perspective-correct, and never feel weird.
-You don't have to worry about animated stuff, if your engine supports skeletal animation, once you craft your base skeletons and animations, you can apply it to different characters/objects with little to no alterations. If your engine supports IK and animation blending, it's even less work to make them feel natural.
And so on. But again, depends on the individual.