As the subject got mentioned on the War & Conquest yahoo group, I thought I would open up a topic here for discussion.
Generally I imagine the rats being reasonably quick on their feet (see the Elves) with the main blocks being 'craven' and so unless they outnumber the enemy, reluctant to attack.
There will be 'the black guard' black furred rats who are much better fighters and morale. These can also provide leaders and other heroes.
White rats can have more 'seer' like qualities
Some rats (or their weapons or magic) can carry or spread disease
I am aware a certain company made the rats quite 'engineer based' and once we have a few basic profiles complete, I am sure the community will create the required variations..
