Generally, I look at:
* COC return (I like to see 10-15% depending on the interest rates and the area)
* Age of the building (the newer the better)
* # of bedrooms (Usually at least 3 is what I like to see)
* rents about 1% of the purchase price
* follows the 50% rule, generally
* property and insurance costs
* high avg income, good jobs and low crime rate in the area (think "nice quiet suburb")
I'm also seeing more the benefit of getting multifamilies rather than SFHs, mostly to get the most bang-for-the-buck out of your loan. I've found smaller mortgages are more costly, more trouble to get, and can quickly max you out.
Generally, I'm suspicious of homes that rent out for 2% of the purchase price. To me, this often means a less desirable area, less potential for appreciation, more vacancy, etc. A cheap deal doesn't usually mean a good deal.
I have a couple of condos, but I definitely wouldn't get more, mostly because HOA fees tend to be unpredictable and they cut into profits too deeply. Also, HOAs can be unfriendly to landlords.
Basically, all of this list is geared toward creating steady retirement income, so I want as much "hands-off no-hassle" as I can get.