Sorry if posting on a topic this old is bad form.
I just wanted to say that I really like this and think it suits the tone of DF pretty well.
One thing that may help it appear more lively is to represent veins of ore running through - this would give you the opportunity to play with color and texture. Also, while I like the color for the flat background, if you could add something to up the visual interest without making it too busy that might help. Lastly, in terms of the individual tiles, you might make them a little more dynamic, so that the lines internal to the tiles carry the eye from one to the next and so on, kind of like text. As it stands the eye sort of seems to get trapped by the pattern overall, but that is just, like, my opinion, man.
A good reference, since it seems to fit the overall style you have going here are some of the more recent side-scrolling Castlevanias - SOTN and the GBA titles in particular.
IN fact, it occurs to me that the Castlevania brand on gothic horror (gothic in the original sense) is particularly suitable for DF due to the inevitable fun you face.
DF is one of my current obsessions and I am mightily disappointed by many of the existing tilesets. I saw that in General Discussions a DF tileset project had been kicked around and I would like to also express my interest in such a project. On the other hand this may be a case of my reach exceeding my grasp.
Incidentally, my inclination is to think that Zelda-style would work better than isometric.
Additionally your mock-up made me think of how well a DF-themed Lemmings would work (particularly appropriate based on how dwarves often act).
Regards,
Edwin