What PCB components typically require hand assembly or are associated with difficult assembly?
Some features I've seen on parts that require hand assembly:


Unbalanced part. If the center of gravity of the part isn't over the parts of it that contact the board, it will fall over before it can be soldered down, unless specially handled.
Low-temperature part. If the body of the part is made of (some types of) plastic, or contains some low-melting point metal, it may not be able to withstand the heat of an automated soldering process. And it will often also have a spec about the maximum time to keep the iron in contact with it during hand soldering.
Mixed mode assembly. If through-hole and SMT parts are mixed, it may be more cost effective, in a small production run, to hand-solder the through-hole parts.