Just a few suggestions : the first few days often dont give the impression that the cataclysm has just happened - no people, no cars about the place etc… and yet the milk is still fresh.
NPCs
I think it would be cool to up the NPC spawn in the first days, then have it trail off back to some reasonable figure by about day 3 (most of them are presumably dead by this point). I had a game recently where my first day was NPC-tastic, and it was a big improvement (albeit random, and frightening).
Vehicles and Broken Vehicles
I also think it would be interesting to see more vehicles about the place, in less specific locations.
Firstly, vehicles randomly on highways would be cool and making driving on highways even scarier. Secondly vehicles randomly on streets would be a nice touch as well.
I think having more vehicles about, while upping the realism, might make the game easier. To counter this, I suggest broken vehicles be added to the spawn.
Broken vehicles could be regular vehicles missing a part, missing wheels, out of fuel, broken windscreens etc. We could also add in ‘dead part’ items like wrecked engines, dead batteries etc. This would make the intact vehicles in garages/lots still precious, but make salvaging a working vehicle from 2-3 wrecked ones a possibility too.
It would also be legit to add in more bicycles.
Dogs
Simple touch - the first 2-3 days could have an elevated dog spawn rate, dropping off to what it is now by about day 3. friendly NPCs (that go on to die horribly) are a good way of making the player feel, ironically, even more alone than he would without allies at all.