How nice of the site moderators and administrator to set up this new low tech forum so that no one can agree what it even is.
For starters, I don't know that it can be exactly defined. Pretty much every set up is unique, and as such it would be hard to draw a hard and fast line. If low tech is less than 2 Watts per gallon and no CO2, what do you do with a tank that has 2.1 WPG and no CO2? I think that it's best, in general, to leave it up to the owner of the tank to determine whether or not it is low tech. Here's my reasoning:
Watts per gallon: an inexact measurement that breaks down at the high and low ends of the scale. Plus it fails to account for efficiency of lighting, reflector, etc.
CO2: There seems to be a lot of disagreement about whether or not it should be used in lower light tanks. In my case, I'm using it on my medium-low (!?!) tech tank because I already had the equipment.
Fertilizer dosing: Every tank has fertilizers added. They just might come in the form of solids, liquids, root tabs, substrate, fish food, etc.
Water changes: Again, there seems to be disagreement about whether or not these are good for low light set ups. And it also brings up the issue of bio-load. How much bio-load is appropriate for low tech.
DIY equipment: Again, this could also be true of high tech. It could be that your DIY equipment is metal halide lighting.
My example of low tech is my old 30 gallon. Lighting was two NO T-8s. Substrate was gravel with laterite and a thin layer of top soil added. I dosed a comprehensive liquid fertilizer once or twice a week, plus root tabs for my swords. I changed 20% of the water every other week. CO2 was a pop bottle reactor, fed into the filter intake. Filtration was just a regular old HOB.