What are the exact Qualifications for being destroyed by the reapers?

I thought it was space flight capability that made one susceptible. But, yeah, Rob's idea that it being specifically mass effect relay use is pretty likely.

But Admiral Hackett in ME3 mentions to Shepard that the Reapers appear to be leaving the Yahg homeworld alone, and we know from ME2 (Lair of the Shadow Broker DLC) that they are a pre-spaceflight species. Being pre-spaceflight does not exclude the ability to process and work with metal.

It should be noted that orbital bombardment,while a tactic of the Reapers, is not exclusive to the Reapers. It is entirely possible that some extinction events catalogued in the Mass Effect series may have been due to organic species instead of Reapers. The Protheans themselves were imperialistic and Javik implies that the Protheans conquered or even wiped out other species that challenged their dominance. It is not outside the realm of possibility that in the past other species and destroyed civilizations on other worlds.

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For "sufficiently advanced" -- at least by ME3, a species would have to be using the mass relays, or using mass effect technology in general. Sovereign mentions in ME1 that the Reapers are cultivating civilizations along the paths they desire. This allows civilizations to learn and harness mass effect technology -- Reaper technology -- in ways that are predicable to the Reapers. Hence, repeating cycles of civilizations discovering mass effect technology, learning and using the mass effect technology in predictable and linear progression, then being destroyed by the Reapers.

Unclear how the Reapers chose which civilizations to destroy/harvest in early cycles before they built the Mass Relays.

Most likely species that have developed intersolar travel would be wiped out. As the next step would be inter galactic travel and use of Relays. So waiting that time might put them to far up.

They probably would have flipped a coin at that point.  If they knew about the Mars ruins then we would have been harvested. If they didn't know about the Mars ruins then we would be long enough away from developing inter solar travel to bypass us for this cycle.

I would say any species that developed some sort of ftl and had managed to attain the ability to leave their own system.