A thought I've wanted to bounce off of LW for a while -- could clothes be a great filter? They allow and encourage on-planet exploration as individuals are able to survive and thrive in climes they're not innately adapted to, and also allow for quick-switching of adaptation for different weather, which seems to aid long-distance communication and trade. While I think that Wikipedia exaggerates slightly when it says that "The wearing of clothing is exclusively a human characteristic" (hermit crabs?), it is nonetheless much rarer than even intelligence or tool-use (and like both of these traits, humans go to extremes). It might not occur to even a very intelligent species to wear something in order to go into space. The technology used just to move around the surface of their own planet might be really clunky and uncomfortable for several generations of development.
A more likely Great Filter would be a lack of distinct climates, seasons, etc. altogether. The problem that would raise isn't that the species would be too stupid to come up with the idea of tools-to-put-on-your-body (and yet smart enough to otherwise be capable of reasoning and tool use?), but that a lack of variation over time and space would discourage the evolution of generalist or adaptive intelligence in the first place. Instead, all life-bearing planets would be dominated by highly niche-specific super-effective super-simple organisms with no real c...
One serious issue for evaluating existential risk is working out whether most of the Great Filter is behind us or in front of us. This relates to the Drake Equation and similar attempts to estimate the frequency of life in an obvious way.
Over the last few years, it has become increasingly apparent that extrasolar planets are common. However, what fraction of these planets lie in their stars habitable zone has still been an open question, primarily because most of our current methods for planet finding easily find planets that are either very large or are very close to their star (ideally both).
A new study, using the data from the Kepler spacecraft, estimates that about a third of all stars similar to the sun have at least one planet in the habitable zone. There are some issues with this estimate, and Phil Plait discusses them at his blog. The estimate has a large amount of variance. The paper actually estimates 34% +/- 14% and the issues that Phil brings up increases the uncertainty in both directions but it seems safe at this point to consider this not being very far off.
One obvious issue from a Fermi perspective is that some systems will likely have multiple planets in this zone. Also, having planets in the habitable zone is clearly not sufficient for life. By the standard estimates for habitable zones, Venus and Mars are both in the habitable zone of the sun. And there may very well be ways for life to arise outside the habitable zone. Moons like Europa and Titan seem to be excellent candidates, and we can't rule out more exotic forms of life in other habitats although that seems not too likely right now.
However, one thing this makes clear: The part of the Great Filter that is behind us that is due to planets not lying inside the habitable zone is small. So the question is, what does this mean for our estimates of how much of the Filter is behind us and how much is in front of us?