The ultimate case brief looks something like this:
Hadley v. Baxendale
You can get "special damages" for consequence of breach which were reasonably foreseeable at the time of contracting. (Shaft broke- delay cost Hadley money- not foreseeable)
Any more than that and you're wasting time and becoming an inefficient writer.
In my opinion, you're not wasting time if adding more details to your brief will help you remeber important information (whether it's because you have it written down or just because you went through the act of writing it). Also in my opinion, since briefs are entirely for your own benefit, it doesn't matter if you're being an efficient writer (aside from it being good practice). I don't think it's fair to say that a one sentance summary of the rule demonstrated by a case is necessarily the ultimate brief that will be adequete for everybody in every situation.
I agree with most of the other posters - there's no one right way to brief cases and finding the style that works best for you will probably take a couple of weeks of experience. It will vary based on your learning style, the complexity of the case, whether the case demonstrates a key concept or just an obscure exception to a key concept, the amount of detail a particular professor wants to hear about a case in class, etc.