1. Were suitable for full suspension bikes.
2. Cost less than about $40.
3. Were actually worth their weight in terms of loadbearing. Most full suspension bike racks -- even the super pricey ones -- are rated only for 10 kg or so.
4a. Most of the weight should NOT be loaded onto the saddle post, but rather downward to the tire.
4b. Vertical supports along the sides to make them compatible with clip-on Ortlieb pannier-bags like the Office Bag. Racks without side supports (there are several out there) would either let the bag flail into the tire, or otherwise -- since usually only one bag was applied -- could lead to torsion forces.
I used to carry textbooks and groceries in a backpack or courrier bag. I often had back pain. I've never again had back pain since using this bike rack, since I carry no extra weight whatsoever on my body while biking.
While I do not intend to do failure testing by loading my bike rack until it gives, I can say that I use it with my Ortlieb Office Bag filled alternately with vegetables and juice from the farmer's market or massive textbooks around town, as well as repeatedly traversing rough terrain with ALL my heavy rock-climbing stuff (60 meter alpine rope, 2 harnesses, carabiners, webbing, 1kg water, food, etc.), and there has been no failure, and no sign of instability or bending, in two years now.
I totally love instructables!! Since reading an article here about coffee roasting long ago, I've been roasting my own fair-trade organic beans for years now! Instructables is my favorite website ever (sharing first with indymedia) -- so I've decided it was time to participate!