As I was reading this thread, I was thinking how A/B/C/D is meaningless by itself for our purposes. You could have an "A" property but it's on the corner of Get Shot st & Get Raped ave. This is actually somewhat common in NYC; there are blocks in medium to bad neighborhoods that are literally a checkerboard of nice new houses mixed with run down dumps that look like they could be crack houses. I doubt you are going to get "A" rent in that situation but you will be paying "A" maintenance and the seller will no doubt try to get an "A" price for such properties.
@William Robison touched on this when he mentioned that a crime statistic should be included. I have found over the last few years in trying to get into investing around the NYC area, that the only areas around here that could cash flow well, are in what I would consider to be rough neighborhoods. This was a much bigger factor than the condition of the buildings. The numbers never worked on any places in areas where I would feel safe to walk around with my girlfriend at night. Thus, investing in the five boroughs is out for me.
As an aside, on one of my property research expeditions in the Bronx, I remember as I was getting off the bus the bus driver stopped me. She gave me a grave look of concern and said "Honey, you do me a favor and make sure you get yourself back on this bus before the sun goes down, you hear me?" Her statement turned out to be very sound advice indeed. I think I checked out about 2 out of the 10 properties I had planned to see before getting the hell out of dodge. One does occasionally need to temper their desire for cash flow against their will to live.
Frankly I don't think a brand new renovated rental (a so-called "A") is going to produce a better return on your investment than a "C," provided they are in the same class of neighborhood - you will just spend more to acquire and maintain them. (And the people who rent those types of places will expect you to go crazy bringing it back to near-new condition in between tenants.)
I think the real trick is finding an area that isn't super gentrified - I hope I am using the term correctly - but yet isn't a horribly dangerous, sketchy neighborhood. Based on months of preliminary research, it seems like the sweet spot for me is going to be B/C properties in terms of the building condition overall. For neighborhoods, I use a numeric scale from 1-10 where 10 is Mr. Roger's Neighborhood and 1 is Mos Eisley Cantina. 5 is about as low as I am willing to even consider. So basically, I am looking for properties between C5 - B7. Anything above B7 is overpriced and not going to cash flow much if at all.
Live in an A10, but rent out B5's.