A possibility I haven't seen really discussed seriously yet is physics as we know it for FTL travel and mass lifting (in other words, how to lift mass on the order of hundreds of millions of tons---the sheer literal mass of humans and the paraphernalia brought with us [1]---out of our gravity well, without devastating side effects) really is pretty close to the limit for the foreseeable future, or it takes far more dedication and united effort than we are currently putting forth to achieve even modest advances towards those goals. If it takes planet-wide coordination to develop the massive array of technologies to reach 0.01c for a generation ship and materials science to make possible a 1 metric ton space elevator, and 1000 years of sustained effort to get there, because the physics and engineering are just that hard, it is debatable that we will start Year 1 of those 1000 years anytime soon.

Another possibility I do not see discussed at length yet is the soft factors override the technological feasibility. The Great Filter could simply be intelligent life cannot so far get its act together enough to cooperate long enough to make it out of their cradle worlds. We ourselves simply do not have the soft tooling, not to speak of the hard engineering, to sustain consistent multi-hundred year and multi-millenia efforts, and none are even remotely on the horizon as yet. Consider: our vocabulary in major world languages simply do not have a word expressing tens or hundreds of thousands of years as a discrete quantity like "millenium" expresses a thousand years [2].

The very fastest human-made object ever is Helios 2 at 221,232 mph [3]. An order of magnitude and a tripling will get us close to 0.01c [4]. The closest rocky planet in the habitable zone we know of is Proxima Centauri b [5], 4.2 light years away. Assume we can accelerate to 0.01c in the same time it took Helios 2 from launch to its fastest point, 13 years, and it takes the same amount of time to decelerate, so 26 years total just to boost and brake. That's nothing compared to the 400+ years in between.

Okay, make it two orders of magnitude and we're talking roughly a single human generation to make the trip one-way. We still don't possess the accuracy and precision in leadership science, not to speak of governance, to say we can cram enough people into a generation ship to be self-sufficient forever, and with the certainty that they don't detonate themselves along the way. If we can't get our act together enough as a species to work out how to co-exist with Romanis for example, or mitigate externalizing/rentier behavior in management in companies (think Romani-lite behavior) around the world for another example, then we sure as hell can't even comprehend what kind of social structures it takes to spread across the nearest exoplanets for hundreds or thousands of years, not to speak of the galaxy in hundreds of millions of years.

The best we can say at this point is even if we can solve the daunting soft and hard problems of terraforming, and cramming millions of people into generation ships, and countless other goals required to envision colonization, it is a one-way trip, and the civilizations set up away from our home system are separate-but-maybe-familiar. The closest experience we have to that are the old colonial periods of the Western European powers, and even then, they had one to two orders of magnitude time delays less than we are going to experience in the foreseeable future with generation ship tech.

I'm a huge Iain Banks The Culture fan. But I recognize how paramecium-level we are in so many sectors of knowledge, craft, and skills, to reach even the first rung towards that kind of vision. I don't want us to kid ourselves on the work we must put in for our future generations to plant the seeds towards that first rung of the long ladder.

[1] https://www.quora.com/Approximately-how-much-does-humanity-w...

[2] https://english.stackexchange.com/questions/331988/is-there-...

[3] http://www.guinnessworldrecords.com/news/2016/7/nasa%E2%80%9...

[4] http://www.wolframalpha.com/input/?i=0.01c%2F221,232+mph

[5] https://en.wikipedia.org/wiki/List_of_nearest_terrestrial_ex...