in no particular order

- durability
- easy on/off
- large main compartment
- 1 or 2 smaller compartment/pocket
- mesh pocket
- floating adjustable lid
- secure, accesible, zippered compartment for small valuables (keys, ipod, digital camera, etc.)

i've sewn up a couple of sets of panniers in the past. the advantages to making your own are that you can build in the exact features you want, have fun doing it, & and save money in the process.

however, to be honest, it's nearly impossible to beat the quality of the better, mass produced panniers. how are you going to radio weld fabric at home? and if you were to pay yourself only minmum wage for all the hours involved, you'd have to charge as much or more as the most expensive bags on the market. my advice would be to avoid purchasing a lock-stitch machine (assuming you don't own one already) until you've made a couple of sets yourself. and as far as the patterns or kits go, probably anyone who is able to sew their own gear also has the inclination & the ability look around at what's available and then make their own design, as you yourself are about to do yourself.

but best of luck on your own project. when you get them finished, post a pic for us