My main qualification is no CO2 at all and no need for CO2. It doesn't have to be low light, it just has to have low enough light to where CO2 is not needed. There is a difference between just being able to grow Java moss and Java Fern and colorful stems, but both can be done in what I consider a low tech tank.
So, low tech also includes lighting. In the past, there were not as many lighting choices so having a MH or T5 setup used to be "high tech" by the nature of the technology was new (and expensive). Now I don't really include lighting type as it seems you can do all without the need for CO2, which is my personal main qualification.
I don't consider adding ferts to bring it past low tech. But, I feel a low tech tank shouldn't require a lot of work, meaning ferts a few times a month would be reasonable.
Filtration can be anything, especially when we are talking nano's. An AC 70, for example, may be low tech compared to an Eheim 2211 but provides much more flow so I don't consider a canister high tech, per say. I have a AC 50 on a tank I consider "low tech", just because I have one, but have a canister on two tanks I consider "low tech".
However, in general, you would normally assume that if someone doesn't invest in lighting, they don't invest in CO2, or a high end filter. That's not always the case though. You see people with no CO2 with a lot of "technology" in high end filters and high end lighting that still is low enough to not use CO2. You also see people who skimp on filtration and lighting that run CO2.
I have also seen tanks that are low light, high CO2, and moss only. I still consider that "high tech", especially with LED lighting, canister, etc. Even though the CO2 isn't needed to prevent algae, it can grow moss very well in lower lighting.
At the end of the day, it's all opinion. As said, for me it has to have CO2 (pressurized). IMO, lighting plays a factor as people usually run more light but I think you can have a "high tech" setup that is low light. I don't personally filters play into this. Well, maybe if you have a bunch of inline stuff and accessories but no CO2, that wouldn't mean high tech, maybe mid-tech, without CO2. Again, just my personal opinion.