There are several steps from start to end of a project implementation, then in supporting the implemented system. If you go through each of the steps, then you would get a indea of the problems and issues you can face.
Examples to add to your list:
a)Customer or business expert can not give full details in one go,
b)Config skills not enough
c)Other related skills – WF, Security, Portal, ESS, MSS etc. not enough
d)Not enough time to implement
f)standard SAP functionality not (i)not enough (ii)not working
g)No (or not a good) Development/Support methodology in the organisation
h)No (not good) tools to monitor progress
You can think of lot more.