User Serviceable - As much as absolutely possible should be repairable by an end-user with basic craft skills and basic training. This means that as many components as possible should be considered easily sourced and low-cost, world-wide.
Deployable to 3rd world or developing nation - No assumed infrastructure. Ideally as a stand-alone device that does not require any existing grid or resources (apart from atmosphere, a little water and something to tie it down to).
Tethered - Something to strong enough to stop it blowing away, light enough not to drag it down. Conductors to transfer energy down and narrow tube or cavity (can be quite narrow) to bleed hydrogen up.
Low Cost - Ideally low cost (sub $100-200) with a rapid return on investment and minimal ongoing costs for maintenance.