A major step forward towards BTS 2004 is the introduction of the Rule Engine which serves to keep the business logic separate from technology (to a certain extent). Though it is somewhat limited in terms of application but the concept has certainly come a long way.
One of the paint points is the maintenance and modification of rules and their various sources in a large server environment. Since the 'Facts' and 'Vocabularies' can derive from a number of soureces such as .Net class, DB, Schemas therefore the maintenance becomes somewhat of a nightmare in the long run (if at all business rules have to change radically). However it is still a charm if the rules are all about changing values e.g. for a rule like 'IF the CustomerSatisfactionIndex is LESS than 100', all we have to change is the value of '100'.
Development and Testing of rules is another issue since there are a number of methods of testing rules e.g. Business Rule Composer GUI, custom test harness; for different methods you need to modify your approach so that you can 'easily' test the rules.
Rules based on Schemas are usually the simplest to test, however rules based on custom classes call for the possible development of FactCreator (any class that follows IFactCreator interface published in Microsoft.RuleEngine.dll)
In synopsis, the idea of rule engine rulez, yet there is some time before a framework for handling Rules establishes.
The important thing is not to stop questioning - Albert Einstein