The way the news has been going I wouldn't be surprised if plastic is a candidate. After a little less than a century of rapid development in petrol-plastics we're starting to figure out the long term effects. But the next 1000 generations may be dealing with the fallout.

I always thought my Pür was great. /s

Maybe it's wisdom.

Every species that might have grown advanced enough, would have gotten over the point of fighting themselves. So they would be wise enough to have something like the Prime Directive in Star Trek (not interfering with less advanced species' until they reach a certain milestone).

sentience. I think it usually immediately leads to suicide

I think that for a technological civilization to rise, you need some things to line up. First, life has to be evolved enough to have animals, beings with a brain. Then, a species has to evolve intellence to become a tool making species. This species also has to become the dominating species on the planet. Meanwhile, extinction events, ice ages, climate change and population bottlenecks are always influencing the evolution process.

This is for me the great filter, to have all these conditions line up perfectly for an intelligent, tool making species to evolve and thrive.

I personally find the kardashev scale a pretty terrible way to measure the success of a civilization. Maybe the most successful life forms don't become technologically obsessed materialists determined to colonize everything habitable and drain the resources of everything else, yknow?

My guess would be self-replicating biological organisms capable of significant rates of mutation.

But then my preferred solution to the paradox as a whole is basically the "nobody tries" idea.

I don't think there's tremendous reason to try to make ones-self detectable at long distances. It's an expenditure of non-trivial resources for an uncertain result. Since there isn't really any robustly sound logic for making the attempt outside of dramatized sci fi stories, I imagine a vanishingly small percentage of occurrences of intelligent life would make a serious, high-powered attempt at any point.

Hard to determine with what we know. We haven't met any other intelligent species which suggests we've passed the filter. Yet, making that conclusion before knowing there are no others to meet is too presumptuous. But, if I were to guess, I'd think the filter is adaptability.

We're superior to animals for being able to use tools, live in radically different climates, and shape every spot on earth into a livable climate. Even on Mars, the moon, and space. How else would a species venture through space if they can't adapt?

That might be too general a concept for the question though.