Well, if that's all that you're using it for, you shouldn't need to upgrade a whole lot.
You can stick with the V-Brakes if you'd like. Disc brakes will be more reliable in wet weather conditions though. I'd recommend the Avid BB7's with the Avid Cleansweep G2 or G3 rotors. If you do this, you'll need new hubs as well. You can usually find some Shimano Deore hubs for a good price (not deore xt, just regular deore).
The fork should do fine since you're not doing any aggressive riding (right?)
Definitely change the stem as noted.
You may want to look into going to a different shifter + derailleur setup if the Sram X4 gives you any problems. It seems to be a hit or miss drivetrain.
Maybe go with a more comfortable seat? Like a WTB sport level saddle.
Go get some Kenda K-Rad tires on there too for your city riding. Much much smoother tire, and will make it easier on you.
Upgrade your cables! I own the 2010 DB Response Comp, and the cables ARE NOT sealed... granted every cable system is not perfectly sealed, it took me one really wet ride to get all sorts of crap inside the housing, which affected the whole bike. Go with Jagwire, or Avid Flak Jacket cables.
If you'd prefer, get some lock on grips.
You could get a lighter seat post, and handlebars if preferred. As well as some lock on grips.
If you do upgrade your fork, you don't need anything heavy duty. I'd go with a Rock Shox Tora. I own the Tora 302 U-Coil and I love it. Very good suspension at 120mm travel. I have yet to see the limits of it as far as bottoming out, and that's with some heavier all mountain riding.
That's just about all that I can think of that would/could need upgrading on that bike for your use. But overall, as is, the bike will do fine for city riding. I would just move to a smoother tire.
Hope this helps.