Like any management job I guess it's based on a mixture of experience and capability. The pursers have to be able to make judgements about relatively minor things, and the ISM has to be able to handle anything that comes up, and do things like try to fix the IFE sysytem when it breaks down. The ISM also has to be good at announcements.
(Having said that, I don't think CX ISMs are quite in the league of some others - I will always remember a (female) KLM ISM-equivalent who when I was having trouble with some electronics in my seat went away and came back with a screwdriver and replacement parts and promptly did a fairly major swapout of the offending parts).
To serve in First the attendants have to be a lot more familiar with the requirements of those customers. Which means, for example, knowing something about wines (and not being fazed by people like
B Watson requesting them to do things like "violently dump" a US$100 bottle of wine into a teapot

), and about the dining habits of different cultures. It is very, very rare to see a standard flight attendant in First Class. They are always Pursers, with a Senior Purser in charge. In business you will find Flight Attendants, but often they are normally more experienced ones.
I understand from talking to some crew that there is a bit of a logjam now in promotions... previously there was a mandatory retirement policy for CX cabin crew at age 40, which meant that there was always an steady stream of openings at the top. That went a few years ago, so the ISMs are getting older and frustrating some of the Senior Pursers
HTH
[This message has been edited by christep (edited Feb 06, 2004).]