Good subject, purdy welds and all that. Sorry to dredge up an old thread! As a TIG welder, bicycle frame builder and newly annointed G/S owner these subjects have been milling around in my head. Basically the issue to consider is load transfer, you want to brace sections, bends, joints that fail or flex under load. The optimal situation for any welded metal frame is for the material to fail. Meaning the tube bends BEFORE the welds fail. You cannot ask for anything more. Many of the 'additions' I see to frames do NOT address the issues, gussets done wrong will make the situation worse. Some should be welded all the way around and others should not, just the nature of welding and the stresses the process leaves behind. Any area that has a tendeny to fail can usually be observed to be missing a triangulation. A rear sub-frame fails because the load is levered, the front/upper mounting area becomes stressed. Under load the thing wants to buckle like a hinge, any support there is purely material, not structure. The obvious solution is to run a brace from the lower mounting point to the rail under the passenger seating area (a la HPN). BUT, that may hinder other processes so you have to compromise. Quite a can of worms.
Anyway, I'll stop babbling. Here's some pics of a bomb-proof bicycle:
...should give a general idea, every joint is gusseted and triangulated to give maximum strength without excess weight. I have been pilfering ideas/ inspiration from the minimalist bracing done on Malindi's bike (they look great, good reports!), the Overland Solutions stuff, and of course the HPN frame mod catalog!! Which is free to download. I'm done ranting, my apologies!