- front AND rear cutting blades, ideally reversible.
Yes. Really limiting to not be able to pull forward and push stuff backwards.
- good bracing between the 3-pt mounting and the rear moldboard
This is overrated IMO. I have a Frontier BB1284, which is made by Gannon Industrial. The blade weighs 1250lbs and is built like a tank, but no supports to the rear. Hydraulic rippers so you don't even have to leave the seat for adjustment. See the pictures, and no, it doesn't get pulled with the tractor in the picture.
- more shanks are better than less (so, 5 is better than 4 in the case of a 5' BB)
This depends. If you have a lot of ripping to do, then it would be a good thing, but if your doing more grading, then the extra rippers may just slow you down more.