I believe the distinction between conceptual, Specification & Implementation perspectives is very very important conceptual level class diagrams is not a s/w entity it is all about what users actually thinks abt the world, it is the concepts in the minds of users e.g. hospital, Customer, Store, Bank etc. more or less these are real entities existing in the real world in time &/or space. there is no distinction b/n attributes & association in a class diagram which itself signify that we are talking abt real world & not s/w entities. Conceptual level class diagram are used during analysis phase of development.
When we are talking about class diagrams with the specification view, we are talking abt s/w entities & we are talking abt what a s/w entity is capable of doing. What are the responsibilities of this s/w entity(class) we are looking after interface of this class. Java beans are a good example of revealing it properties through introspection it is telling the JSP Container that it can tell the outside world abt its properties but not how.from a spec. perspective there is no distinction b/n generalization & realization, in fact generalization is one way of implementing realization.only public methods are drawn in a spec. pers. class doagram. It also implies the responsibilty of updating relationship with associated objects, if navigability is also present then who has the responsibility of updating relationship.
From impl pers. a class diagram include fields & public, private, protected methods.Now we are talking abt code & other nitty gritty.
Hope this helps
IBM Certified OOAD with UML(79%)
IBM Certified XML Developer(68%)
Sun Certified JAva Programmer(85%)