Just my thinking
Helpdesk
cats as prio and status needs to be structured. Stay as is
Forum
A toss up. Should the users be ablle to create their own cat
Ideas
ideas should probably open to user category creation. Instead of restricting ideas to preconcieved categories, let the user create their own. It isnt like an idea needs to be structured perfectly.
CRM
Could pretty much be tag only, unless you believe a one off all encompassing classification is a good filter to have eg 'Computer Services', 'Financial Services' ect