I've never been very interested in single player maps, but I was somehow compelled to give this one a try. It's really good, of course. The cave ceiling scene blew me away, easily outshining everything else for its correct proportions.
I don't know if you're still working on this, but if you are I say you should make the trees closer to real scale, for obvious reasons (aside from the fact that your design compliments the change perfectly), and certainly make some of the unscalable rock ledges bigger as well, at least until we can't see over them.
And the zombies cease to be scary very quickly. To resolve this, maybe you could have them randomly lunge with great speed once in a while, as they did in the Resident Evil series (the only one of its kind I'm familiar with). Unless later zombies begin to do this...and if not, it might be even cooler to have only later zombies lunge.
Anyway, despite the issues I have with it, its originality alone (or perhaps its level of faithfulness) is enough to make it great. And it would seem that it ages very well, as well. Well let me just say that your grasp on the mechanics of a proper adventure game has been made known.