Check out if the building has any outstanding building/housing violations.

Another issue with co-ops is often related to financing. Mortgages sometimes can be tricky to obtain. My first step in regards to finance is to go to propertyshark.com and see where other people who purchased apartments in this particular building go their mortages (this information is free as well). Generally speaking potential buyers should be aware on what the maintanance payments are, how often they have been raised, fully understand operating expenses of the building, know if the building charges a flip tax on purchases and sales and know whether the building allows sublets. Even if you never plan to sublet the apartment, the sublet policy greatly affects the value of an apartment.

Once the apartment is purchased, conducting the renovations can be an issue as well. I always recommend potential buyers to do a preemptive check. Before purchasing the apartment, get in touch with the building management and ask them what their requirements are. If the management puts enough hurdles, I feel that it is enough of a reason to walk away from that particular apartment.

Always visit the area at night. What you love at 3pm may not be so wonderful at 10pm.