Re: Opinions please.

I don't think its a terrible thing, though it could be a little confusing. If someone looks at your proposal and they see "... is immune to red cards" they may then be more then a little confused when there is no red card for the unit to be immune to.
Another small problem is that it could lead to an exploit later on. People are probably pretty likely to forget about abilities that are effectively useless when they are defined. Then later on someone could design some type of exploit that uses the fact that people might have forgotten. For example using your red card example, lets say someone defines an item that says "select a unit you control and a unit controlled by an opponent, the selected units may not be issued any commands for 3 days." People might consider this a fair item assuming they forgot that your unit is immune to red cards. This was the first example I could come up with but it is a little bit of a concern.
If an ability like this were the only bad thing in a proposal I would probably still vote for it but if I was wavering over the proposal it might be enough to make me vote against instead of for.