From my experience characteristics of a good POS system should be:
1.Compatibility with all the versions of a platform,for example in today's scenario a good POS software,if made for windows platform then it should be compatible with the latest windows 7,vista till windows XP.
2.It should be able to function even when internet is down,storing data offline and updating online whenever it gets the connection.
3.Should be smart enough to be accessed through variety of interfaces like website,touch screen, mobile etc.
4.Should give alerts on mobile and email on critical situations like quantity falling below reorder level.
I request others to correct me if i am wrong and even add to it,so that this forum becomes a solid knowledge base for all prospective POS software users.